This takes place in a magical world with 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, 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:
- 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.
- 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.
- Soul is bound indefinitely to the necromancer's will, but preserve their identity.
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?
The mechanic I've decided I'll be using:
After the summon, the undead now have physical body, and 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).
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.
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.
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.