# Keeping a world with necromancy from overpopulation

This takes place in a magical world with a medieval setting. They've undergone an apocalypse (not defined yet, but it has no relation with necromancy) that wiped all but few thousand survivors, all permanently scarred from the tragical event that left humanity hopeless.

With so many casualties and so few survivors, a few necromancers decided to use their dark art to bring back the dead to help with the rebuilding of humanity. At first, of course even the survivors shunned this act, however they began to accept the use of the undeads, and even learned simple necromancy to command the undead to help with everyday duties.

A few hundred years later, necromancy had very much become a "religion". People teach and practice it everyday, even the young ones are taught necromancy - they can even resurrect dead pets!

Necromancy has advanced greatly due to the unrestricted research:

1. Specific soul can be bound, but it needs the original body to find the correct soul. You can bind a random soul to a random body, though.
2. You can bind a soul to any body, but the extent of its abilities are limited to the damage it sustained, i.e skeletons cannot move at all because they don't have muscles.
3. Souls are bound indefinitely to the necromancer's will, but preserve their identities.

People bringing the loved ones who just died, kids raising their old dogs, and students reanimating their excellent teacher.

Because practically all people can practice necromancy, it can be concluded that at some point the world will be overpopulated (not that undeads need food, but space).

How can I explain that it will never happen?

UPDATE:

Thanks for all the answers! So many great ideas, and I'm having difficulty which one to pick, as each one is interesting. I'm going with combining answers from Oak, Paul, Mr. M, and Adwaenyth.

The mechanic I've decided I'll be using:

1. After the summon, the undead now have a physical body, and a soul body. This soul body moves the physical body (so I don't have to explain the dead brain, or if the brain or head is missing).

2. The physical body will still rot away and can be damaged. The previous rule still apply: if in process the feet go missing, for example, then it can't walk.

3. There's a "soul pool", but is unknown to the inhabitants of the world. Basically, reanimating takes a random soul from the afterlife (the pool), then binds it to the physical body.

4. There will be a fight against an (currently undecided) entity - I'm thinking of the cause of the first apocalypse, but that will be too cliche.

• What happens to the undead when their commanding necromancer dies? – Alexander Jun 27 '17 at 18:11
• Their soul will be unbound from the body, and can be used by another necromancer. – Vylix Jun 27 '17 at 18:34
• Given that the undead need recent bodies and not food, I don't think you will have problem for millennia. The Earth could support quadrillions of "people" if space is the only problem. – user2617804 Jun 28 '17 at 1:24
• The question has been answered sufficiently, but I still want to point out two things: a) no kid wants to reanimate their dead teacher b) nobody wants a corpse around them all the time either and c) this isn't as much of a problem unless the undead reproduce. Who would reanimate their great-grand parents, some that most people never got to know? The equilibrium should be very manageable – Raditz_35 Jun 28 '17 at 11:32
• @Vylix If you like it so much, here is more: There were some South American people (I cannot research what happened there exactly atm to give you more details) that paraded around the corpses of their loved ones. They didn't end up with more corpses than they could handle. While they weren't alive, they were treated as if they were. I think this is the closest you can get and overpopulation still no problem. – Raditz_35 Jun 28 '17 at 12:02

Your problem is impossible to limit without going too meta. Some of the suggestions while good, simply impose arbitrary limits on your necromancers.

They either restrict by demanding a 1 for 1 trade (kill one human, bring back someone else) or by limiting the number of times someone can perform the art. I propose something different that gives you some freedom:

As you've mentioned that the bodies stop deteriorating due to magic, perhaps make it not be the case. The "ressurected" body will not deteriorate per se, instead its "animal molecules" will slowly be replaced by magical ones. What this means is that there is effectively no deterioration noticeable. The issue comes with that the body ends up at a point where it's sustained only by magic, and as soon as the link between necromancer and "body" is broken, the magic "releases" the body, thus turning it into a pile of ash.

Technically you can have overpopulation at the beggining (everyone will bring back everyone), but in practice you can only bring someone back once, because you "consume" the body when you ressurect it.

• I think it's a cool idea! Think of this like "Impure World Resurrection" jutsu, but slowly the body deteriorates (not preserved). I will use this one, thank you! – Vylix Jun 29 '17 at 8:21
• @Vylix You can also make it so the longer someone controls a body the more power he needs to control it, as they would be sustaining more "magic molecules" to keep the body intact – Oak Jun 30 '17 at 19:24

Put a limit on how many undead creatures each living person can control. When someone is at their limit and wants to create more undead, they need to release some of those they already control. Released undeads could either die immediately or lose their preservation, which means they will become feral while slowly rotting away.

When a living person dies, all the undead they control get released, unless they perform some ritual which transfers control to someone else (who needs to have enough free capacity, of course).

Necromancers who have more practice, better knowledge or are born with more innate talent might be able to control more undeads.

• I like the idea of leveling up. You might even be able to rip the mechanics of it out of an existing RPG system to base the story on. – corsiKa Jun 27 '17 at 23:24
• @corsiKa If the story is being embedded in a video game, this will surely be a great opportunity to meld between the story and mechanics. – Lawful Lazy Jun 28 '17 at 1:26
• Ah, the ol' "additional pylons" mechanic. See Starcraft or Warcraft for unit control cost. – user1306322 Jun 28 '17 at 6:36
• The problem is that as the number of living increases the limit to the number of undead increases too. A possibility could be to set this limit in the legal framework of the living instead of as an intrinsic limit to the spell. That way, your society can self-regulate it, just as we regulate our retirement age in relation with our demography. – Aaron Jun 28 '17 at 15:59
• Undeads becoming feral as suggested might lead to a zombie apocalypse, which may be an interesting plot twist although not very original. – Pere Jun 29 '17 at 7:54

One simple solution is that which you find in games: mana. Make the use of magic a limited resource, and there you have a limit on the number of undead. More experienced/knowledgeable users can use more mana and use more undead, but are still limited. The non-undead will rot, and you won't even have that many bodies to put your magic on.

it can be concluded that at some point the world will be overpopulated

Of course this is to answer your question. But one thing that strikes me that nobody else commented yet, is on your main concern. You asked how to avoid a problem and people are telling you how to do so, but nobody commented on the problem itself. Overpopulation? Let's think about it for a minute.

"wiped all but few thousand survivors"

Earth is right now at 7 billion people, and if we migrate to sustainable means of production, we can hold much more. There are extensive parts of land with no use at all currently. So, to overpopulate, specially with beings that take space but don't need food, clothes or consume anything else, we could maybe triple that or more. Not even counting that you could keep your undead in the ocean.

Assuming a world similar to earth in size and land area, and a population of only a few thousand, it would take several thousands of years to repopulate all of it and reach the current state, in terms of population. And that is assuming an non-conservative growth. Then you add a few other factors:

• when someone dies, their respective undead go back to rotting until someone else takes over (Google says Maggots can consume up to 60 percent of a human body in under seven days, meaning if people don't act fast, that undead is lost forever)
• plus the natural wearing out of the undeads in use
• plus wars where massive numbers of undead gets destroyed

Keeping the supply replenished starts to look like a challenge. Even if you don't follow any other of the suggestions here to limit the usage of necromancy and such, we're still looking at several thousands of years of having enough space for everyone, living and undead.

How can I explain that it will never happen?

When you take into account all those factors, they don't explain that it will "never happen", but for sure it is not a problem to be worried about for uncountable generations (more than 100 for sure). And if the world is medieval, they will most probably evolve their technologies quite a lot, very fast. Specially with so much free labor. It's very feasible they will reach the space age in less than 2 millenia.

Maybe ask yourself how relevant to your story it is to worry about that at this point. ;-)

Note to self: a civilization with space exploration technology and necromancy. Sounds original, this idea can go somewhere. X-D

• re: "civilization with space exploration technology and necromancy" Endymion comes to mind – Azor Ahai -him- Jun 28 '17 at 23:53

You could solve the problem if you are open to some wiggle room on your 4th point: The undead body is preserved by magic.

If the undead body doesn't regenerate from all damage or that regeneration has a cost for the summoner, then the oldest or most hard used corpses would eventually be unable to function or might simply be not worth repairing compared to replacing it with fresher specimens.

• I might actually take this approach. Having a perpetually preserved undead seems too overpowered. However, I'll wait for a few days before accepting the answer. – Vylix Jun 28 '17 at 8:29

Well in the best stories that involve magic, there is always a cost or a catch to the use of magic. That is where your mechanism for not being over-run by zombies lies.

1. Have the dead being bound to a body, even their own original body, something that the dead don't really want. They want to rest, so that each day is a test of wills. Maybe make it easier if it's a close family member in a loving relationship. The results don't have to be violent or gruesome, but if the soul wins, the corpse just collapses. This kind of breaks number 3 in the list, but that rule makes necromancy way too powerful.

2. A soul that has won it's freedom from binding can not be bound again.

3. The longer someone is dead, the harder it is to bind their souls.

4. There is no guarantee that the dead will inhabit a body peacefully. In the process of binding, the departed may want to strike at the necromancer, creating a chance of insanity.

5. The higher order a creature, the harder it is to reanimate. A kid can bring back a goldfish, a teenager the family cat. A twenty year old can bring back the Dog.

Talent and natural ability can help with this as well. Only the really skilled can raise and maintain more than 6 or 7 undead at a time. Fill your asylums with those who tried to bind the wrong soul to a body. Maybe do away with the total preservation of the corpse and simply have it last twice as long.

These are some example rules you can impose. Introduce some risk and cost to the power to keep it from getting out of hand.

• I like the 2nd point. Make it a bound soul cannot be reanimated the second time. – Vylix Jun 27 '17 at 21:58

Make the undead unable to give new life. Basically raised people can have sex, but they can't have children.

Now you might think that this still makes for a bigger population and you are correct. But it is not that huge of a difference. Math is on your side.

To make calculations easier, we'll assume that child mortality is not an issue, people form lifelong couples and the ratio of men:women is exactly 1:1. We'll compare a world where necromancy doesn't exist, with the one where it does with the above conditions.

let x0 be the number of people immediately after the apocalypse
let c > 2 be the average number of children per pair
let n be the number of generations we will look into the future

Then the number of people without necromancy is:

And the number of people with necromancy is:

Which is a simple geometric progression sum, hence:

Now lets examine how fast z grows compared to y:

This means that with 3 children per family you will get up to 3 times more people with necromancy than without. With 4 children - up to 2. With 5 children - up to 1.67. And so on. Here is some code if you want to play around with different values.

Now, 3 times more people might sound like a lot, but it's really not.

## tl;dr in case you don't care to follow the math:

If 3000 people start reproducing and don't use necromancy, no matter how many generations you look ahead, there will always be more people than if 1000 people start reproducing and raise every single person that dies, given there are 3 children per family on average. If there are more than 3 - the numbers are even more in favour of the no-necromancy folks. Evident by this graph.

However, this required us to ignore infant mortality. One way to deal with it is to make raising babies/small children illegal and/or frowned upon. This doesn't look that far fetched given:

1. The undead body is preserved by magic.

My interpretation is that raised people don't get older and don't grow. Hence no one will want a couple to look after a crying infant for the rest of their lives and they will be nudged to try to conceive again instead.

But even if you raise infants, the conclusion will still hold. It just won't be 3000 vs 1000, but 10 000 vs 1000 for example.

• One could also assume that a world with necromancy would have a slightly smaller birth rate, compared to a world without necromancy, since one of the main reasons for having children in the medieval era was such that they could help their parents on the farm, and eventually care for them when they got old; with necromancy that's not a problem, since all you dead grandparents can do that just as well as your children -- or better, since they are more experienced. – Nikolaj Jun 28 '17 at 18:40

# Kill off the living

I'd base myself on the World Of Stone from the Death Gate Cycle by Weiss and Hickman.

Basically, reviving someone also requires someone else to die.

Since you can make your actual world bigger than the known world to your characters, they don't have to notice a one-on-one connection. Reviving someone in Magical Midieval London can kill someone in Magical But Not Yet Necromancy-Converted South-America.

The few people that do die in the near vicinity of the necromancers, could look like they've been hit by a mysterious disease. They would just be resurrected anyway.

By the time the people find out, they might be too dependent on necromancy already. At that point you might rest assured that someone decides that a few deaths on the other side of the world are acceptable in return for cheap labor. The working class and their unions are a bother anyway, no?

Of course, this might require you to change the way your magic works, which may or may not be allowed.

• Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus Jun 28 '17 at 6:17
• +1 This is actually interesting approach, even though it cannot be used in my story - if there's a life sacrifice people would still forbid it by now. – Vylix Jun 28 '17 at 8:27
• Assuming that the undead can do necromancy themself: Go further and kill all the living people, live forever with the undead you already have and resurrect / repair them when needed. Population is fix, Problem solved. – Henning M. Jun 30 '17 at 15:12
• @HenningM. I like your way of thinking ;) – Vylix Jun 30 '17 at 19:51

You might to ask yourself some questions:

First of all, how many people would be on the planet if procreation continued as normal?

Only about 120 billion people in total ever lived on earth. Of earths surface roughly 55 million km² are arable land (no deserts, no mountain ranges, etc.). 120 billion / 55 million ~ 2200 people / km². That is -mind you- with our earth's population growth rate. That is that of a averagely populated suburban area.

Of these 120 million bodies, how many would be in "serviceable" condition?

Considering that any body only 30 years before that event has become dust already. Since your setting is a medieval timeline, that makes about half of the 120 billion corpses already unavailable - resulting in 1100 people/km².

Since population got drastically reduced during the event, how would population develop into modern times?

Most of the population growth occured in recent times - and is exponentially. Since our starting point reduced the people capable of reproduction down to a few thousands, even with medical advancements, we can scrape off the gigantic amount added by recent generations.

Additionally, with so many corpses available, people might do something... what would in reality be very frowned upon, further reducing the birth rate, or simply not see the reason for having more children.

Also in war, reanimated corpses might be totally annihilated, partially prohibiting the resurrection of said corpses.

Thus we could easily scrape off 90% of the remaining 60 billion people due to these effects and come down to 600 million living people and 5-6 billion reanimated corpses without further ado. Which is about the same population that we currently have in real life.

So - unless you explicitly need an overpopulation problem in your world as a plot point - you might not even run into an overpopulation problem in the current generation. It would however occur in future generations.

There are a few other avenues to consider:

## Being undead isn't fun, and you may be next so you treat your undead with respect

Eventually everyone dies. Being reanimated isn't life - you may be able to control your body insomuch as your necromancer allows you to, but your blood isn't pumping, you can't eat, and most of the pleasures and joy of mortality is removed for most people. Depending on the damage you might not have the same mental capacity (brain damage starts occurring within minutes of death), nor strength (muscle damage, rigor mortis), ability to move, and the possibility (likelihood?) that your sense will be dulled, and possibly not available at all.

Even the best case, where your death didn't impact these things and your body was reanimated immediately, the undead will still miss out on the pleasures of food (no moving blood + no working organs = no working digestive system), sleep, etc.

It will drive an undead batty, the necromancer will have to exert more control, greater restrictions, and eventually the "soul", inasmuch as there was one, will no longer be present.

This leads respectful necromancers to wonder - what will happen to me when I'm dead? What do I want to have happen to me? Even in the absence of religion, it's very likely that people will have strong feelings regarding how long one should keep an undead around, and no doubt it will be a thought strongly on the minds of those who are undead, wondering how long they are going to be forced to live this way, and how long until they no longer have any real control over their death.

## Religious proscriptions

Likely the problem of overpopulation will have cropped up early on, and for a variety of reasons the strong religious institution that teaches necromancy to all also teaches moderation and control. Perhaps the elders of the church will require that they be consulted for each raising - of course for purposes of time they either 1) allow quick raising, but then license must be obtained, or 2) everyone is given license, but if someone operates outside the rules their license is revoked. Those rules can be quite arbitrary, but it helps if there's a historical reason for each one.

## Necormancers lose control when they die

Another option is that the magic keeping undead alive dies with the necromancer that raised the undead. This can be further strengthened if you disallow multiple necromancy - the first to raise is the controller, and that undead cannot be raised again if they are released or if their necromancer dies. This means the undead will live no further than one generation beyond their own, and thus the possible world population will never increase past 2 times the natural population (ie, people will live a possible lifespan of no more than 2x their normal lifespan).

Could have some interesting implications - for instance the wealthy will always hire very young but competent necromancers to stay with them 24/7 so they can 1) be raised immediately after death, losing the least amount of physical/mental capability and 2) live the longest. The necromancer is then set for life, and in turn they may become very wealthy, hire their own necromancer, and so on. You may have very interesting caste/class systems due to this mechanic.

That's an interesting setting.

Specially this bit: "Soul is bound indefinitely to the necromancer's will, but preserve their identity."

Which I understand that the undead keep their personality after reanimation and are able to communicate. And have meaningful conversations and requests.
Like "It's time for me to go, my dear". Which is a hard request to deny coming from a family member, knowing that you will very likely be reanimated as well and would like to have the same wish respected if/when you request it.

I think that the amount of undead will regulate by itself and undead overpopulation won't be more of a concern than living overpopulation.

It would be treated as a way to increase longevity for the most part. You'll have most of the population reanimated for a while and then liberated (aka unbinded and cremated). Some won't. Some will like to hang around for centuries, chaining reanimations (I see some aristocrat families taking pride of having 200+ years old members).

You see I'm not worried about forced reanimation. It's because I think it would be quickly seen as something very dickish to do and revolutions would be fought to make it highly illegal.

• Yep. Chaining reanimation is what was my first thought when I come up with this story. Hence my first implication that comes to mind is overpopulation. – Vylix Jun 28 '17 at 17:20

Well, first let's consider how we interpret magic. From various descriptions I have read it could be suggested that there are roughly two perspectives on the nature of magic ( not to construct an arbitrary dichotomy here, but merely for the purpose of illustration ).

One such view is that of enforced balance, i.e. magic is constrained by, let us say, "meta" laws of physics. This can also be seen as a creationist perspective or spiritual basis for the existence of magic, i.e. it just is because God said so.

On the other hand, magic can be seen as a reinterpretation of the natural laws of physics. From this point of view balance is an abstract concept which applies in some hypothetically infinite universe, and therefore, locally, magical power is only constrained by one's knowledge of the natural laws. So in other words magical power ( and not magic itself ) is constrained by ignorance. This can be seen as the agnostic perspective, magic exists because the universe exists, we don't know why but clearly the magical and the physical are made of the same "stuff", each effects the other and both follow the same rules ( assuming we understand those rules ).

One could argue of course that both perspectives have truth to them.

EDIT: it was not clear whether the force described is adverse to the process of transporting souls based on the previous description, the below is reworded for clarity.

Now iff, from the first perspective, your magic imposes a cosmic balance on the prospect of extracting souls from one "place" and depositing them in another, then one could describe a normalizing force or gradient which acts to prevent souls from being displaced from one plane of existence and deposited in another. The imbalance could be described as sort of capacitance between the planes of existence which would suggest ( given the normalizing constraint ) that the planes of existence are finite, or that the physics which separates the planes has limits - the latter being more palatable from a physical perspective as the former would not make much sense cosmically since we tend to think of the universe as infinite.

However, iff, from the second perspective, your magic has no cosmic force which limits the movement of souls in such a way, i.e. the "capacitance" of the magical imbalance is infinite from a local perspective, then the only thing which limits the movement of souls from place to place is human sentiment, a simple cost-benefit analysis that humans compute intuitively and act on collectively as a society like ants with emergent behaviors.

Given these two perspectives and your needs for the story, you could certainly combine aspects of both kinds of constraints to make the rules as complex or as simple as you like.

Limit total soul count

Let's say something along the lines of... when people die, their souls are just swimming around in some kind of huge, eternal pool, where they wait to be reborn as someone else.

This will completely limit the possible amount of humans in the world at one time.

Now if the souls get bound to their former bodies through necromancy, it will decrease the number of souls that are ready to be reborn. So less and less people will get pregnant (though I guess the number of actual people is pretty low anyway).

This will prevent overpopulation from ever happening and at this point in time, nobody would frown upon necromancy since the "soul pool" is still big enough to expand humanity a lot.

Although I guess there's still the question if people even know about this whole system or are just blindly using necromancy 'cause it's so darn convenient (as people do).

No need to explain that it will never happen

Now this is a different approach which I thought I'd at least mention... since when do humans care?

Sure, some do, but humankind as a collective generally doesn't. It's kind of like you're trying to write a story about our world and then say... nah stop, we still have to explain away overpopulation, global warming, etc. because otherwise people wouldn't keep living the way they do.

• Welcome to WorldBuilding.SE Mr M! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus Jun 28 '17 at 14:22
• This is by far the best answer. And for your second point, sure, but we are explaining it to the reader, not to the inhabitants of the world, but via. – Vylix Jun 28 '17 at 17:17

### The undead body is preserved by magic.

And that's the key. Treat magic as an almost-infinite resource easy to obtain but hard to replenish. Is a common setup in magic universes that "magic" is a consequence of the existence of living beigns. In other words, just imagine that "mana" is available in the whole world, but is more common or concentrated around places where life is more common as well, forests, big cities, etc...

On the other hand, living corpses uses magic for living; it could be a delicate ballance between living beigns spreading magic as they live and undead beighs consuming magic as long as they're living. Create too many undead bodies and they wil deplete the magic of the zone quickly, and after that they will die (again) like my cell phone when it run out of battery. Or create just the right amount of undead bodies and they will consume the ambient magic at the same pace as it is replenished.

If the ratio of magic creation vs magic consumption is 1:1 (one living being spreads enough magic to sustain one unliving being) you tie the amount of undead bodies to the amount of living bodies so, if in some point of time the undead outnumber the living ones, they wouldn't last for long.

Life and death are tightly related. It's obvious that one cannot exist without the other.

### Nature don't like undeads, but tolerate them.

It is not natural for a dead being to be alive, every cell in the body of the undead is craving for resting forever. Well, you can cheat nature using magic but, the longer you cheat mother nature, the more it costs.

If the magic cost per day follows the graphic below:

For each day of existence (x axis), the magic cost increases (y axis), so after 100 days of unliving, dayly cost is almost twice as the first day and 200 days after the cost is multiplied by 8.

No matter how powerful the Necromancer would be, there would be a moment where the magic costs exceeds Necromancers power and this body cannot be kept living anymore.

Maybe you can use both ideas at the same time! :)

You should think about how many dead are really there to be reanimated?

Scenario 1: slowly growing living population (1% living population growth / generation):

It takes about 70 generations for the living population to double, so looking back less than 10 generations, the living population seems constant. If animated dead go back fully dead after the controlling necromancer dies, the corpses would be unusable damaged after a few generations have passed. So at the very worst case, the full population would be about 10 times the living population.

Note, that it is a constant multiplier! This means, the overpopulation problem would not be caused by the undead, but by the living, since the whole population is N times the living population.

This means, if an overpopulation ever happens with the undead, it would happen a few year later even without the undead.

Also note that raising bones would not take up much place, you could have probably 100 generation worth of dead relatives in the cellar.

Scenario 2: rapidly growing population (100% living growth / generation, each family has 4 children on average):

To simplify, let's suppose, only the "children" are living, parents, grandparents, etc are all dead. This means, if the number of living is N right now, the prev. generation had N/2 people, N/4 before that... So right now there are about as many people living as there are all the dead! (of course that nasty war throws off this calculation, making the dead population relatively bigger for a short time, but as generations pass, this will cause less and less error in the calculation).

So basically, if your population is growing very fast, the number of people living now won't be much less than the number of people that are dead. So raising every single dead would probably double the population.

Once more, the overpopulation is not caused by raising the dead, but the exploding number of living.

Actually, on the contrary, in a fast-growing population the dead would be a scarcity, a resource hardly acquired, and fought over...

• "war throws off this calculation, making the dead population relatively bigger". It makes sense to me that, if there was a war, the first ones to be at the front would be the undead, not the living; only after you've exhaused your useful undeads you would start using living people. So I'd argue that a war actually consumes your undead population more that it increases it. Besides, as living people in the front die, they may be used as undead in the same front, reinforcing this idea. ;) – msb Jun 28 '17 at 22:14
• @msb With that remark, I specifically thought about that big war before necromancy became mainstream, the one that almost wiped out the population! - that made a huge dent in the exponential population growth counting from that world's Adam and Eve, so I felt it should mention. however, you are right, wars surely has an effect on the population, which I did not take into account. Although, wars are either just slowing things down, or they are the major means of living-population control! – Zoltan K. Jun 30 '17 at 13:02

# Necromancy Mastery (Limit)

The most easy way could be that your necromancers has a limit of undead that they can control. For example a boy could control 1 or 2 animals, a teenager 3 or 4 animals, a young man 1 or 2 person, a normal user of necromancy 3, and a necromancer teacher 5 or 7.
By this way you can have a limit and also you can train it, like levels in a game

# Life "Glue"

In order to bind the soul to the death-body you need a way to stablish that link, you use your own life force to do that. A "casual" user won't note a difference, but maybe a normal user has 1 or 2 years less of lifespan and a proffessional 5 less years. By this way people has to chooce between comodity (undear servant and workers) and lifespan (more years).

Maybe you could do that death-bodies are always rotting, slowly, but rotting, maybe it could take 10, 30 or 100 years, but they can't be in our world.

The dark magic that use your necromancers "stop" the aging and rotting of your undeads, but in the moment that the necromancer die, all the aging and rotting of the corpses is released instantly, so the undeads would be destroyed and nobody would be able to ressurect them.

# Corpse Time Limit

You can only ressurect people after some days of die, if you have a undead who was revived 5 years ago then when you die any necromancer would be able to revive them, because the corpse is "too old".

Undead souls are difficult to manipulate, if they are to much years in our world they are more stronger and when the necromancer isn't able to controll the they can become agressive, if you have an agressive undead police could command you to destroy it for the your own good and the good of your neighbours.

# One-use-souls

Simple, undead souls can be only "tamed" one time. If the necromancer die they can be "tamed" by other necromancer (maybe powerful necromancers could transfer the control to other one).

# Life Balance

This isn't my idea, I only want to express my agree with the DonFusili answer. Recently, one week or two ago I saw something similar in a series.

# Self Vanishing

You said that undeads preserve their identity, so they after some years (20, 50, 100 or a little more) they don't want to "live" anymore, they are bored of living so much or the think that it's bad, so they will request to the necromancer to free they souls (if a dear person is asking you to release I think that you won't put much resistance, it's they own choice, not your).
Also undead persons aren't able to enjoy mortals feelings, they only work without stop, they would't they release earlier or later.

# Law Vanishing

Think about yourself, after die, What do you want to your body and soul? Do you want to be an slave for the eternity? Maybe alive persons would make some "moral" laws about the years that an undead could be in our world. They will have rights! Maybe religion determine that laws. Also you could need a licence to use undeads, the licence has it own limit of undeads and time for each one, also if you do something bad you could lose your licence.

Here’s a novel answer: make the number of souls in an area limited to the number of people who used to live there. Whatever the historical greatest population was of a town, that's the total number it can be brought back up to via necromancy.

This has the twist of limiting reanimated persons geographically and can cause complications when travelling.

• But then I should've be able to explain why it's geographically tied to an area? Heh, I can start thinking why it is possible. – Vylix Jun 29 '17 at 8:46

A consequence of overpopulation might be food shortages, unemployment or perhaps plagues, right? But the undead may not have to eat, work or be capable of carrying disease (though I like concept of a civilization threatened by reanimated cadavers wandering around).

Remember, overpopulation may be a perfectly fine thing to have in this world, so long as the problems or opportunities it would present are written away from the challenges of your protagonists.

Instead of trying to find a cool fun solution, you could just use economics. How do you prevent the world from being overpopulated with the walking dead? Some asshole figures out how to make money off of the space needed to care for a dead person, and then keeping dead people around gets too expensive. You can also solve the issue of people not using this asshole's service with social pressure ie "You keep your dead cat with your living cat!?, what are you poor/dumb/evil".

This is a petty answer, but not ever problem needs to be solved with magic, even if it was made by magic.

Maybe instead of putting limits on undead, make some difficulties for the living - excessive use of necromancy makes one impotent. That way people can either command army of undead or have living family. It can fit nicely in Your idea of "necromancy as a religion", where most powerful necromancers can not have a family.