Why isn't a magical obligate parasite living in a wizard library exterminated?

This is related to my last question here.

The library here is not necessarily the magical sentient library mentioned in the previous question, but can be any wizard library. This library can be in any form e.g. a secluded tower in a wood, or a huge library where many wizards live together, or in an academy.

This parasite is an obligate parasite: it cannot live outside a wizard library, or it will die in 2-3 days. The parasite can be found in virtually any wizard library, and even an apprentice mage can detect its presence.

Considering it is a parasite, and it often tampers with a wizard's experiment or spell casting:

Why do wizards not attempt to exterminate these parasites?

1. The parasite feeds on remnants of magic power used from experiments or spell casting. Often, it intentionally distracts the caster so that the spell fails, to get more magic remnants.
2. There is no mana required to cast a spell. The residue of magic power is only produced after spell casting.
3. There is no benefit for a wizard or mage to keep this creature around.
4. This creature can accidentally activate scrolls. However, it is not believed that it does so on purpose. It might be on purpose, but it has never been proven. Scrolls can be activated without knowing the inner workings​ of the spell.

The physical or magical features of this creature are up to you to decide, but I'm imagining a ghost-like semi-corporeal creature flying around the library. You can make up a fully incorporeal ghost, or something like that.

• Comments are not for extended discussion; this conversation has been moved to chat. – James May 23 '18 at 16:48

Regarding:

There is no benefit for a wizard or mage to keep this creature around.

I disagree.

The parasite feeds on remnants of magic power used from experiments or spell casting. Often, it intentionally distracts the caster so that the spell fails, to get more magic remnants.

This parasite can be used as a magic residue cleaning device - even the apprentice knows that magic residue left uncleaned can wreak havoc.

This creature is a living training dummy. A spellcaster needs to maintain concentration and cast a spell properly even if something is biting his leg or whispering nonsense in his ears.

This creature may be useful for exterminating other vermin like rats, mice and cockroaches.

It may keep apprentices awake when they perform Sleepless Night Meditations.

It can even scare off trespassers of wizards library - unlike magic apprentices, trespassers are not aware that this scary noises, shadows and ghostly monsters are just illusions created by cat sized magic residue eating creature and most damage it can do to human is few bites and scratches.

Probably it looks like domesticated ferret:

This creature may be simply cute.

So spellcasters keep it as a mascot.

• Comments are not for extended discussion; this conversation has been moved to chat. – James May 23 '18 at 16:48
• Easy. They can't.

Why are rats not exterminated easily? Cockroaches? Pigeons (in big cities, I remember an anecdote about one city that had a population of 50k pigeons, killed 120k over a few years, maybe one or two decades and afterwards had a population of 50k pigeons)?

• They respawn / are unkillable

• They are generated by magic, come from another dimension, lay eggs (or similar) in books that can't be detected and therefore infest libraries

• It is too much effort, especiually related to how fast they come back or how much damage they actually make.

• Maybe there is only one big parasite but a million very small children that can only get big if the big one is killed, so killing the big one will just get you a few days of pause, then another one turns up.

Or, to add insult to injury, it might be a kind of initiation. Maybe the older, more experienced mages that would be able to remove those parasites had to learn to cast shield spells while casting their other spells which is a very important ability and therefore they want their novices to learn that too. So they just don't remove that annoying little parasite that feeds of the magic residues which else could be used as shortcuts for new spells.

• We will often eradicate pest from single buildings, even if we don't remove them from cities in their entirety. Maybe, if these things die when they are weak (old age, starvation, illness) that's fine, but if they die when they are full of magical energy (you try to exterminate them) they set off a nuke. – Shane Mar 20 '17 at 16:39
• @Shane all those chemicals in a library? An extermination spell wandering through magical tomes? I think hadnwaving (or otherwise explaining) why that's simply not possible should be easy. Plus, I provided some reasons in the answer already. But yes of course, pest control is not impossible for us - in a well defined and suitable environment. – DonQuiKong Mar 21 '17 at 8:50
• No, it said 2-3 days outside a library, where there is no magical residue. How long does it take the parasite to consume all of the average magical residue in a library? You would need to shutdown the library for that amount of time plus 2-3 days. (and additional time if any accidental scroll activations add magical residue.) Perhaps that could add weeks. Also "found in virtually any wizard library" implies it easily travels to newly built libraries - so now you may be talking about shutting down every wizard library in the nation or world for that entire time, otherwise quickly re-infected – LeBleu Mar 21 '17 at 20:14
• I was going to suggest this, adding the point that killing them can probably only be accomplished by magic, and any such killing must - inevitably - add magical residue from both the eradication spell and from the death of the residue-eating creature. So you'd be making the situation worse. It'd be like trying to eradicate roaches by simply squishing the ones you can see with a heavy lump of sugar, then leaving their corpse and sugarlump there for other roaches to eat. – Dewi Morgan Mar 21 '17 at 21:41
• @LeBleu Yeah, that would be the worst case scenario. And it is still would be easily manageable IF the desire to eliminate these creatures was strong enough. IOW, it points more strongly to 'eliminating costs don't outweigh the problem they cause' rather than 'we can't kill them'. – Shane Mar 23 '17 at 17:10

The librarians like them.

Wizards don't like the creatures, but librarians like them.

For one thing, it keeps the wizards from casting spells in the library. Any librarian will tell you that spell-casting is a fire hazard! And what happens after a tired wizard has "just tried to heat my coffee"? Right, their efforts to stop the fire will do more damage than the fire.

Give me an honest parasite any day! It enforces the "no magic in the library" rule better than any librarian.

Also, keeping too many books of magic in one place is dangerous in itself. These creatures keep the magic buildup from becoming too bad.

Yes, they eat the occasional scroll, but it turns out that these by pure accident are the very scrolls librarians don't want wizards to read, like Fooldare's Unquenchable Fire and Stilgar's Infinite Fountain.

And when there is a magical accident, as will always happen when wizards are around, the creature will help in the cleanup.

• I love this answer. Especially liked the "These creatures keep the magic buildup from becoming too bad." and the tongue-in-cheek "(...) by pure accident tehy eat the very scrolls librarians don't want wizards to read," – xDaizu Mar 20 '17 at 15:31
• Books like water even less than they like fire. [Citation needed] – xDaizu Mar 20 '17 at 15:31
• One other reason the librarians might like them: the parasites know the library better than any single librarian is capable of knowing it, and can occasionally be contacted for help. (Ever since one imprudent Acting Chief Librarian cast Infinite-Dimensional Bookcase to ease a temporary shelving crisis, this has been a life-saver.) – jez Mar 20 '17 at 19:56
• My books like water much more than they like fire. If treated well, they dry off and are as good as new (although perhaps a bit wobbly). If burned, they blacken and vaporise. – wizzwizz4 Mar 21 '17 at 19:25
• Edited to remove water references, since they were not central to the point. – Stig Hemmer Mar 22 '17 at 8:39

Temptation:

Why are they allowed to stay? The wizards hope they can get the magic-eaters to change.

Nobody in your world has read this Worldbuilding thread. Nobody knows you have decreed these creatures are permanently and forever useless. All the wizards see are creatures that sometimes eat magic. Think of the possibilities! Just think of them! If they could be tamed…

The wizard who tames these creatures will have power like nobody has ever seen.

Want to try an impossibly complicated spell but worried it could get out of control and backfire? With friendly magic-eaters around waiting to serve as magical ground wires absorbing undesirable magic spikes, why not take a chance? A wizard with this kind of safety valve will be able to try more, learn faster, experience fewer debilitating setbacks. That sort of tool would be immediately useful to any wizard at any skill level.

Rival wizard causing problems? Send some of your tame magic-eaters over with instructions to really ruin that jerk’s day.

The possibilities mentioned by the other posts in this thread may have also occurred to your world’s wizards.

Rationalization:

The potential for gain is so tempting, nobody has made a serious attempt at finding a permanent method of keeping them out of the libraries. Your wizards probably rationalize the havoc they cause in all sorts of ways… ”The magic-eater ruined your spell again? What are you complaining about, apprentice? Back when I was your age, the magic eaters we had were twice as hungry! And we survived! I tell you, you apprentices these days are just too soft. You want to be half the wizard I am, you’ll learn how to cast those spells with magic-eaters stuffed up each nostril!”

Urban Legends:

No wizard has yet accomplished these lofty dreams, but many wizards are secretive and proud and haughty. Nobody is willing to admit they have absolutely no idea how to control these creatures. So, much like the real words to Louie Louie, every wizard knows someone who knows someone who’s friend heard that someone in another library said that they know the secret. And so wizards, nervous that they are actually falling behind their fellows in the race to ultimate power, lie through their teeth and brag about all the successes they have had. Any day now, they will have the technique perfected! Honest! And the boasting generates more urban legends. And so on.

The Common Manaphage, is but one of a large family of such species. It out-competes its relatives, and while a Common Manaphage remains present, no other could get a toe-hold. Having a long-time resident Common Manaphage basically garentees you are safe from infestations of other more exotic breeds. Some of these exotic breeds are hard to differentiate from the Common Manaphage, so it is good to know you have one, to save having to worry about checking the identity of any that show up.

The other breeds include:

• The Maneating Manaphage; which while thoughout most of its lifetime eats only background waste mana, lays its eggs inside a wizard, which the hatchlings will devour.
• The Malodorous Manaphage; which skunklike emits a putrid odour, both as a defense mechanism, and as a technique to distract wizards into providing it with more food.
• The Magpie Manaphage; which collects shiny objects, like wands, cystal balls etc, to decorate its nests in the high and dark corners of the library.
• The Multing Manaphage; sheds its oily black skin/feathers nearly continously; ruining books and carpet.
• The Manuscript Munching Manaphage; which is known to supplement its diet with rich vellum and other papers.

Luckily the Common Manaphage out-competes these more problematic species, consuming the resources that they would otherwise need to grow, breed and thrive. So the option becomes: deal with its minor problems constantly. Or every few months exterminate the new one; knowing that one day a dreaded Greater High Eldritch Manaphage may turnup/grow to maturity, and cause you never to have problems again.

• +1 for "manaphage" (though "thaumophage" could also work nicely!) :) – Dewi Morgan Mar 21 '17 at 21:43
• I like "thaumophage" that is great. But it breaks all my alliteration. – Lyndon White Mar 21 '17 at 23:31

They're afraid of the consequences

Magical libraries are ecosystems that evolved over millennia. (Otherwise, you wouldn't have an endemic species in it). Removing a life form from that ecosystem can change it radically, upsetting the fragile balance (because, with magic involved, any balance is fragile).

A lot of things can go wrong:

• Your parasite is part of a food chain; what happens when the animals that used to feed of the parasite's dead bodies suddenly go hungry? Will they start eating librarians? Or worse, the valuable scrolls?
• What happens to the things the parasite ate, besides magic? They might be harmless now, but overpopulation and absence of predators might change that.
• Magical scrolls have a natural immune system, and fighting the parasite trains and strengthens their defense mechanisms. Removing the parasite will make the scrolls vulnerable to magical attacks.
• Extinguishing the parasite will open an evolutionary niche - and something will fill that niche sooner or later. That's just how evolution works. Something that might be much more dangerous than the original parasite.
• This is a good answer if you consider the ecological niches occupied by similar such “pest” vermin of our world. Whether the asker's world was designed by some deist Maker or has remuted from a cosmic spore, perhaps the wizards would be hesitant to exterminate an entire genus of animal for much the same reason as conservationists of our world. – can-ned_food Mar 21 '17 at 1:31

The first thing to spring to mind was, as Vodolaz said:

This parasite can be used as magic residue cleaning device.

and would be the answer I like most if it wasn't for your specific instruction that the parasite can't be useful. So either we cannot get rid of it, it is dangerous to do so...or we don't want to for some illogical reason.

Can't get rid of it

Using magic to rid yourself of a magic-hungry pest could be dangerous...perhaps this was once attempted but the pest learns to use any spells which are cast on it.

It is dangerous to get rid of it

They eat up excess magic so perhaps destroying them would release all of this - causing a volatile state of raw mana which can spontaneously create spells you can't predict.

Some illogical reason we cannot

When creating a spell the mana is channeled through the user, perhaps after absorbing excess mana from their spell the creature looks like that person - meaning your library has a few ghostly apparitions resembling previous great mages. There could even be a debate as to whether the parasites take on more than the looks but even some of the personality of stronger mages who channeled mana through more of their core being.

(Personal opinion but I like the last one most)

They like it.

Each library has its parasite, which has been there a long time. Each one has its particular quirks and manner of appearing to the users of the library. It is the library mascot. Wizards who have come up using a particular library find the immortal parasite endearing, reminding them of times past and younger days.

I could imagine the parasite is sentient and communicates with the wizards, when it chooses or if it is begging. It is never malicious and it is fond of his library and its users also, once it gets to know them. It is funny and grumpy. It sometimes can be helpful, especially in exchange for treats. It has been known to gorge to the point of illness. It has been known to growl at intruders and fend off threats to its home, the library, including one particular episode it will tell you about, many times. It might remember something of use or something that happened long ago, albeit unreliably so. As regards spoiling spells it is like the lovable golden retriever whom you know will steal from the counter if he thinks he can. Or the great uncle whom you know will eat any frosting left unguarded in the refrigerator. You get mad when it happens but you do not throw them out.

A parasite of this sort can be a real character and offers narrative possibilities.

• And it's not a coincidence that every library's parasite is "charming" -- its a defense mechanism that they acquired to keep from being eradicated. The malicious and dangerous ones were wiped out; only the cute ones remain. – Shawn V. Wilson Mar 20 '17 at 1:59
• Cat. Although it's not considered as parasite, its egoistical moves have been known well and sometimes considered disruptive. I immediately thought of cat when thinking about this creature. – Vylix Mar 20 '17 at 3:17
• They could be referred to as "library souls". Nothing says that wizards cannot have their own superstitions and beliefs (like in marine). Also, if wizards would allow only for likeable (for them) parasites to reside or only such creatures would spawn, they could reflect character of their master wizard. That's why even wizard students could find parasites to be funny creatures. – evar Mar 20 '17 at 12:33
• In reference to @AdiNugroho - maybe the parasites are like "magic fleas" and like earthly fleas, like to infest cats, and perhaps that's the reason (or part of) why cats are such a classic "wizard's familiar". Interesting angle... ;) – JVC Mar 20 '17 at 17:19

Student: I was just in the library practicing a new spell, and this thing bit me! It was about <this> long and...

Master: hold on, you were practicing in the library?

Student: yeah?

Master: You were TALKING in a library? In the library where grand masters have studied for generations?

Student: ... yes?

Master: My student. Talking. In a library.

Student: ...

Master: What sort of manners did your mother teach you? Why do you think they shush you all the time! There's no talking in the library! Everyone knows that! You've only yourself to blame! No talking in the library!

Student: I... I'm sorry master.

Master: You're going to go to the head librarian right now, you're going to apologise to her, and then you're going to ask to be tested for manaworms.

Student: I will, master.

Master: oh and if you ever see one of those things again...

Student: Yes, master?

Master: put it under a glass and bring it back here. They make the most scrumptious cakes.

To expand on the suggestion by DonQuiKong that magic may generate the parasites:

Parasites are a known side-effect of mixing magic residues. Build up enough magical residue in any one place, and the chaotic interactions of the residues creates things. They can be killed with magic but doing so only leaves more residue, creating more problems. The best solution is to wait for the magical background to subside, which will starve and weaken magical parasites.

Within a large magical library, seepage from the magical codices and scrolls can sometimes be enough by itself to spawn parasites even when no spells have been cast.

Because of the chaotic nature of their creation, each magical parasite is also unique and unpredictable. Trying to cast spells on them can have unwanted effects and has not been allowed since the incident where the head necromancer's late apprentice cast a fire spell at one.

• I like this! An endless cycle because this thing spawned when you cast spell, and you can only get rid of this by spell. – Vylix Mar 20 '17 at 1:17
• Patience, tolerance... and the occasional move to a cleaner library while the background magical radiation fades in the old mage's tower, abandoned apart from the occasional foolish adventurer. – Dewi Morgan Mar 21 '17 at 21:45

Because it is the magic of the library that spawns it,

The parasite is a direct consequence of putting that much magic and knowledge about magic in close proximity of each other. You cannot eradicate it because for that you have to eradicate the magic itself.

It is no more strange than that if you put lots of fissile material in close proximity of each other, you get some interesting and useful effects, or in some cases disastrous effects, or even hideously destructive effects.

Another way of looking at it: even though the constituent elements that make up our bodies — carbon, nitrogen, oxygen, phosphorous, calcium and all the others — cannot form anything interesting while on their own and separated, when put together just right, they form a sentient, conscious, bipedal being that is capable of uttering things like "I think, therefore I am". The same with this magic: on their own, little fragments of magic cannot do much. But when you bring them together... things happen.

• Or, perhaps, you could eradicate that one... but the nature of the magic means you'd have another one waiting tomorrow. – Joel Coehoorn Mar 22 '17 at 14:25

The common imp (AKA fairy, brownie, or domovoi) are innately magical creatures, found in any location where there are (or have been) large accumulations of magical power. While they are not made of magic themselves they do require it to live, and are remarkably adept at manipulating it to their own ends, having been observed teleporting, becoming invisible, and exhibiting remarkable telekinetic and illusory abilities.

These creatures are inherently destructive, though they have evolved a remarkable survival strategy to ensure they are never fully eradicated.

Using the magical forces at play around them imps have developed the ability to manipulate the minds of those around them, forming illusions that convince nearby wizards (and mundanes) that they are not dangerous. Those affected are under the impression that the creatures are helpful in some way, are beautiful, or can be bribed into performing tasks (which is rarely true).

The effects of the imp's magic wear off once the wizard is out of their immediate vicinity, and with suitable training or good luck a wizard may be able to realise they have an imp problem, however once they return to eradicate the imps they once again fall under the spell.

Luckily imps can only draw so much power from their surroundings, and often work at cross-purposes to one another (imps are naturally argumentative and aggressive). This means that large swarms lose some of their potent manipulative power and can be dealt with in whatever manner the wizard sees fit. There is, however, a chance that some will be left, and after a short period of hiding they will re-emerge, once again convincing any nearby that they are no threat even as they wreak havoc.

If no reproductive pair is left (imps are hermaphroditic) then one singularly powerful imp will remain. These creatures will have such a hold on the mind of any nearby that often they will be actively defended by people in the house, and some examples from eastern Europe have seen witches attempting to kill those who threatened to disturb the imp living in their house. These 'domovoi' sometimes take the image of a person they often see as an extra layer of defence.

Magical libraries are famed for imp infestations, and often require specialised extermination teams to periodically clean up the fairy gatherings, brownie haunts and imp nests that naturally accumulate both inside and nearby.

They keep specimens around for research.

These parasites are presumably something of a societal problem. Researchers working on dealing with this issue would likely want a few specimen around to study.

I'm reminded of a bedbug expert who kept a jar full of them. To feed them, he'd open the jar and stick it on his arm for several minutes.

I hope this answer was helpful. I have to go puke now.

1. Balance

In a world where magic casters could free internal forces making things more chaotic parasites are then returning nature to its initial charged state, which seems like them "eating "those forces. Societies that succeded with local deparasitation was choked by overwhelming residual magic chaos, leaving dead magicless ruins behind. So parasites and libraries are protected by the law now. You can think their importance in magic world as importance of plants in oxygen/carbon dioxide world

2. Scent

Ability to sense magic make them equivalents to magic hounds. Since they can not leave the library, they can't chase mages, but if you give one some parts from place where powerfull magic was ised, it could lead you to magic tome with corresponding spell helping to identify it.

It's against their code of ethics

Since the enlightenment, all (respectable) magicians take the equivalent of the hippocratic oath (an ancient principle commonly summarized as "do no harm") before learning their first spell. Naturally, in times of crisis, the rules of this oath may be "bent" a little... but killing to avoid a minor inconvenience is beneath them.

• Interesting first answer. Welcome to WorldBuilding! Could you provide some more details regarding the hippocratic oath? Most people are probably familiar with the term, but some might not and links can go out-of-date, which is why we encourage to summarize the contents of a provided link in a few sentences. If you got questions take the tour and visit the help center. With 5 reputation you can also ask on Meta and with 20 you can visit the chat. Have fun! – Secespitus Mar 21 '17 at 15:31

Because the creatures always take away wizard's will to kill it.

The parasites can have a defense mechanism from the wizards that threaten them in their natural habitat. And being magical, they can bypass the unneeded stages like looking nice and cute, defending by physical means, etc. and go directly after the source of the harm - the very intent in the wizard mind.

So every time a wizard thinks that he's got enough of these pests, 2 options are possible:

• Either the desire to kill it evaporates instantly, ending in nothing more than talk,
• Or the desire is there, but the wizard can't actually bring himself to do anything about it, lacking any willpower to act.

I think you should go with the idea of having an old grumpy wizard, very important and powerful, who would kill anyone who tries to get rid of the poor little things.

Have a grumpy leader guy love the things with no reason and everyone else thinks he's crazy, but they respect him.

• I like this idea, but I think you could flesh it out a bit. Are there any exampels where something like this has been done in fiction? Any real world reference where something similar works (like a boss at work)? Right now the answer is valid, but a bit short. – Secespitus Mar 24 '17 at 9:59

The creature adapts to the knowledge around it.

One can imagine a magical creature that, as a defense mechanism, instinctively absorbs the magical knowledge of its surroundings to be able to counteract spells cast upon it. (Think Borg, but magical instead of scientific.) Of course, a library is supposed to be a repository of as much knowledge as possible, meaning only obscure, exotic, secret or brand new spells can affect it since those are less likely to be found in the libraries.

This also explains why it's primarily found in libraries; few other places have enough information for the parasite to be able to defend against even weak/apprentice wizards.

They're unkillable unless you stop using (and storing) magic.

For me the most obvious feature of this magical creature is that it's created by (and evidently feeds on) magic. Ergo, any attempt to kill it with magic is simply going to feed it and make it stronger. The stronger the spell used, the more you're feeding it.

It probably likes people using powerful spells to try to kill it, hence why it keeps annoying wizards. Eventually they'll snap and try (pointlessly) to Avada-Kedavra! it, providing it with a delicious feast.

Because, like the library, the parasites are sentient. One does not commit genocide on sentient beings that one has caused to be brought into being through one's actions, unless one wishes to fill one's library with evil magic residue.

And if one happens to be an evil mage, who WANTs evil magic residue, then doing so regularly would make more evil residue, so killing all of them off would be counterproductive.

Also, being sentient, they are also vindictive. Most of the old tales say not to mess in the affairs of the elves, or spriggans, sprites, or other fae. That's basically what these things are. Sentient, spiteful and capricious.

Leave milk out for them each night, and a little bread, and you'll be fine.

For the same reason we haven't eradicated malaria.

Namely: cost and ecological concerns. Perhaps exterminating these parasites is very expensive, and so it's cheaper to just sacrifice a scroll every now and then.

Or perhaps they are an endangered species and are protected by the international convention on wizard culture preservation. Or perhaps no one has ever had a really large wizard library without parasites, and they are afraid of what might happen.

Moreover, if smaller libraries don't have parasites, then perhaps they are a sign of prestige.

Analogous background on the mundane war against parasites:

Malaria costs the world economy billions of dollars and hundreds of millions of lives every year. We have had the technology to end malaria for decades. In fact, malaria was extirpated from the united states in the 1950's. However, that old technology was too expensive to be used effectively in rural africa. Furthermore it had an unintended ecological impact (see DDT).

Today, we have technologies like sterile insect technique with gene drive that could quickly and cheaply eradicate the handful of mosquito species that transmit malaria (and zika, west nile, lyme disease, missouri encephalitis, rocky mountain spotted fever, etc.) with almost no unintended side-effects.

Currently we choose not to end these plagues because people are uncomfortable with intentionally extirpating any non-microscopic species (even one or two of the thousands of species of mosquito or tick).

Many people are also categorically opposed to any form of genetic engineering, especially the poverty-stricken people who could most benefit from it.

Both of these concerns should wane away over the next few generations so that our grand children can live lives free from the fear of deadly hemorrhagic fevers and chronic zoonotic illnesses.

I'm thinking these are ghost cats.

• They are soulless killing machines like a normal cat, but they are ethereal like a ghost.
• They can only survive in a library where they feed on leftover magic just like domestic cats can only survive in an urban environment where they feed on leftover human food.
• Wizards tolerate them because they keep the ghost rat population in check.
• Ghost rats can survive anywhere and occasionally chew up enchantments in the wild, but in high-magic environments they reproduce like rabbits and can consume all the scrolls in the library in a matter of weeks.
• Obviously, when a ghost cat walks across a scroll you're writing, you get annoyed, but some wizards actually learn to love the ghost cats who become a mascot of sorts for their library.

Side note: there are rumors that ghost rats transmit spotted wizard ague, but some wizards think it's actually the ghost mites they carry to blame.

They made humans magical, but none knows details

Humans were an unmagical species in a world full of residual magic, left over perhaps by the elves... These creatures transform magic into biological existence, releasing a tiny fraction of magically saturated air. Upon inhaling that, humans slowly develop magical abilities. Nobody knows this, but it has been observed that the presence of the Grindlequark (or whatever it is called) greatly benefits spell casters.

It is even rumoured that total isolation from grindlequarks for one year and one day renders one unmagical. This is all the more true of magically imbued artefacts, which abound in the library.

Unfortunately for some dark wizards, the practice of preparing Tumbleweed-Grindlequark stew has not enhanced magical power in any palpable manner. But many still cherish it for its reputation and thick, savoury quality.

Therefore the library not only doesn't dream of exterminating the Grindlequark, but novices are actually assigned to individual Grindlequarks, vouching for their safety and beckoning to their every whim. The creatures have proved remarkably adept in discrediting novices who don't guess the wishes they communicate through guestures and grimacing...

If there is a competing race, eager to render humans unmagical again, they might even kill Grindlequarks in the wild, so the keeping of sanctuaries would be all the more important.

1. They can't get rid of it. The wizards try, but for some reason the best they can do is banish it for a few days, or knock it out of action for a while. After that the ghost will reform... or a new parasite will hatch out of its ashes, phoenix-like. Their magic simply isn't strong enough to kill it off forever.
2. There is more than one parasite. Dealing with the parasite is like us putting a flea-collar on the cat. It'll kill a lot of fleas, but there are plenty more hiding in odd corners of your house, or living as eggs or larvae in the carpet, just waiting to recolonise your cat as soon as the chemicals in the flea collar have lost their potence.
3. The parasite is actually a commensal or symbiote - it has beneficial as well as detrimental effects. It may mess up spells now and then, but it also preserves magical books from attack by insects and mice nibbling on them, or makes certain types of spell easier to cast, or acts as a guard dog to the library.
• The third is not plausible, as it is already stated that this thing does not actually benefit wizard. But I like how everyone else mention that it can be cleaning service for wizard. – Vylix Mar 20 '17 at 1:15

By interfering with some spells, the parasite makes it very dangerous to cast spells that might harm the library.

Over the years, some magicians have exterminated the parasites in their libraries. These magicians (or their apprentices) invariably went on to cast spells that destroyed their libraries.

Thus, all of the remaining magical libraries have these parasites. And their resident magicians are very cautious when casting spells on the library grounds.

Side note: The refugee magicians are not wanted at any other libraries (not wise enough, don'cha know), so they wander the countryside. When they sojourn in cities, they put out signs saying, "Will cast spells for food." This begging gets them into trouble with the local mages guilds.

• This could be an interesting answer if you emphasize the correlation between the question and your answer. – Vylix Mar 22 '17 at 2:16

These things are like magical cockroaches. You see one but you know darn well you've probably got hundreds. Could you exterminate them? Probably but at what cost?

You might get rid of the largest, most active pests, and it'd take an archmage to completely clean a library - until the next book with a dormant one living in it comes in.

A good reason why you wouldn't get rid of them is serious extermination risks doing more damage than the parasites would do.

For example: You need serious poisons or fire which damages or destroys delicate scrolls.

If this creature is spawned from the remnants of magic, it might suffice to say that this creature cannot be killed or destroyed by any means other than magic.

This means that within your world, if the wizards attempt to use magic to destroy the creature, they will be creating more magic remnants which will simply create more of the creature.

Since this is a perpetual cycle, the wizards have found that it is simply best to leave the creature alone, even though it is a nuisance.

This also opens up some fun possibilities for older wizards to challenge the youth:

"If you can figure out how to cast a spell that doesn't leave enough remnants for the creature to appear again - ... insert plot twist"

Make them literally be ghosts of the people's ancestors, and nobody will want to kill them, otherwise they will have opened the doors to people killing them once they die and become a parasite too.

• I think it would be nice if you could flesh out that answer of yours a bit. Right now it's a bit short. Is there any religious or sci-fi reference you used to come up with this that someone could find out more about this? What would be the implications of this for the people? Would they be more careful when visiting the library, etc.? – Secespitus Mar 24 '17 at 10:01
• There's no religious or sci-fi reference, it's just an idea I thought up on the bus. As for implications etc, I leave that to the OP to figure out if they want to advance the idea, I'm just providing justification for not destroying them, the rest is not my concern. – Callum Bradbury Mar 24 '17 at 11:53

The consequences of killing it may be worse than leaving it alone.

If you ever have run over a skunk, you'll know what I mean. Of course, to be a significant deterrent, either this must not be one of a kind and/or the bad consequences set in (and alarm the librarians) long before the creature/infestation actually is in danger of dying.