Nemesis Fiend

... The Sentry chose the name "Nemesis fiend" not out of misplaced scaremongering, ladies and gentlemen. This girl knows you. Inside out. She knows every weakness, because she is born from them. She sits on the other side of the veil and tries to manipulate you, such that you would become more similar to her, until finally she can come over, kill you and replace you. And then... do things.
And she knows which strings to pull. Hence the name. It is not what she will do... and trust me that is bad enough. It is because she wants to make you do things. This girl is a nemesis.
And it is your job not to let her become too powerful. Thats why you are here. Thats why we can't let you go anymore. Because you are not ready yet. And you have to become ready, before she does... Now about that protective circle..."
"So... uhm... it is always female?"
asked a young student from the back. Ashar didn't notice who, as she was using an ocher stone to paint a red circle on the floor. Male voice though. Lifting her head and blinking for a second about the, from her perspective, stupid question, she gathered where it was coming from and sighed...
"No, of course not, gender doesn't even mean anything to it, if it can just turn you. This speech had just gotten.... a little personal to me. But if your question was hopeful... better watch out. Yours might be, but it's a bitch you absolutely do not want in your bed...".

— Ashar Nirudi giving one of her infamous speeches during "class" in the tower of Amn.

The Nemesis Fiend, Origin and Agenda

Not all demons hail from other Fae realms, some are born in the Counterrealm itself. One of them is the Nemesis Fiend. Once arcane Spellcasters learn to manipulate Fae external to their own, their subconsciousness and their personal fae also begins to form a much closer bond to the external Fae, thus "opening" them up.
This pollution of the personal fae with external fae initiates the formation of a nemesis fiend, a Fae creature based on a casters worst aspects, his subdued wishes and desires, and all those things he always wanted to live out but never found the courage to.
Like everything Fae, once it gains enough complexity, it also gains life and, eventually, self awareness. And just as the casters desires influence it through the Veil , so will its desires influence the caster. Wizards and other arcane spellcasters have theorized, that this connection originates from the fact that the nemesis fiend was born from parts of the personal Fae of the spellcaster, and that it is actually the fiend, who is the relay between personal and collective Fae, through which it is possible to cast using collective fae at all.
Be that as it may, by exploiting this connection, the Nemesis fiend slowly begins to manipulate the caster. Usually, it attempts to enhance the way he feels the very urges, that birthed the nemesis fiend in the first place, yet which he may now follow even less, for if he did too often, the distinction between him and the fiend would finally become blurry, opening the path for the nemesis fiend to take him over, destroy the casters soul and turn him into a new Plaguefiend.
A caster whose time of "transition" is approaching, becomes easily irritated and twitchy, sometimes even antisocial and violent. This is when the caster KNOWS that he has to pass through the Veil, face his Nemesis Fiend and banish it, which is ,sadly, always temporary, because a new, but similar, nemesis fiend will start to form soon.

Banishing the Nemesis Fiend

Due to the personal connection between caster and nemesis fiend, it has to be the caster who does that. Although spellcasters could theoretically hunt down nemesis fiends of other spellcasters, from a practical point of view they wouldn't even know which demon is another casters nemesis fiend, or a nemesis fiend at all.
And even if they would, because of the connection which nemesis fiend and caster share, every wound brought upon the fiend translates into a wound which "his" caster suffers too, if it was not him who struck it. "Killing" another casters nemesis fiend in the counterrealm thus provides an effective, and unverifiable way to also kill the caster. And yes... that is a technic assassins would exploit, if they were able to find the damn thing.
Because of all this, the only one equipped to face his nemesis fiend and banish it, is the caster himself. He can find it, and he can "kill" it.
Humans, due to their connection to the Fae, occasionally pass halfway through the Veil in nightmares that are called faedreams, and sometimes they get a glance at their nemesis fiend, when the time of transition draws near. This means however, that it is high time to banish it, because it showing itself just like this also means that it feels safe enough to challenge the caster if need be.
Most spellcasters however do not wait this long, and "clear their register" early, and on a regular basis, when the nemesis fiend is still weak and far from ready to pass through. But this is also not without danger though. The process of passing through the Veil by mind travel opens a path between the Counterrealm and the body of the caster, and the nemesis fiend is not the only demon which is waiting for a chance to exploit this.
The reason why demons from all kinds of Fae realms are often found lurking in the counterrealm, is, that every caster who comes here to face his nemesis, provides them with a chance to gain their so much desired ticket to the other side. Therefore each trip through the Veil is dangerous. Because, once a demon, be it now nemesis fiend or another, manages to kidnap a caster's body and closes the connection while he is mind traveling, his soul is left stranded in the counterrealm, which is a fate considerably worse than death.
Not a few demons who haunt the counterrealm today, have once been the souls of trapped casters, changed by the constant flow of Fae into terrible parodies of what they once were, and now trying to "return" to Ardu to live out "their" life, by stealing another casters body and perpetutate the cycle.

Cultural reception

The Name "Nemesis fiend" was deliberately chosen by the wizards. It would have earned this name already by its terrible agenda, which inevitably spells doom for "its" caster, provided he does nothing about it. Yet, because it comes from the caster and basically represents his supressed urges, it also knows him especially well, which makes it even easier for it to manipulate him. Hence the name "Nemesis". It is a very personal foe.
All this establishes a race for new casters to participate in. A race for their life. Once they have managed to cast their first spell, the race is on and they must finish their training, and develop the mental tools, to overcome the insufflations of demons and their own nemesis fiend, before the fiend becomes too powerful and takes them over. Facing their nemesis fiend, and overcoming it for the first time, thus usually also indicates the end of their training.
This is typically also the only time when a caster meets it in its fully developed form, something he later, for good reason, always tries to prevent.
Hence, a spellcasters apprenticeship always comes with a predetermined runtime. And because no culture on Ardu embraces the creation of Plaguefiends, the last step in their career is typically witnessed by those who have passed the test already, and who come in the company of warriors who can end the life of a rising plaguefiend, in case the aspiring spellcaster fails.

Towers

In order to limit the dangers that arcane spellcasters pose by threatening to turn into plaguefiends, humans have tried to codify their training in Towers.
Originally established by the Church of the Creator, the ideas of the towers is to provide central and controlled places to not only to train the casters, but also filter those out, who have no chance of surviving the confrontation with their nemesis fiend, and take them out of the equation before it is too late.
Originally this was a job for the Inquisitors of the Church, yet after the rebellions on R'hyll another organization was established, which served the same purpose, but which does not demand membership in a particular religion: The Sentry. Today, except in countries in which the Inquisition is still in charge of this task, they keep an eye on the casters who are trained in each tower, and hunt down any plaguefiends and -beasts in the regions for which their chapter takes responsibility.
In most cases The Sentry and it's Sentinels are in charge of a tower. An exception to this are the towers of Amn, Soth-Kessuth and High Haven. Amn, because here Magistra Ashar Nirudi holds the reigns, much to the chagrin of The Sentry and the Church, Soth-Kessuth because even though the Sentry is supposed to hold power, they have become mere executioners rather than what they are supposed to be, and High Haven because in Haven, the center of the faith in the Creator, the Inquisition still performs their job.
Together with the experienced magisters of the tower, the Sentry (or whoever is in charge) decides if an aspiring wizard has a chance to overcome his own nemesis fiend and tries to estimate how much time still remains until the casters nemesis threatens to overcome him. This is done early, because the creation of a plaguefiend always enhances the chance of regional plague outbreaks in the short run, and also furthers them in the long run, because each plaguefiend that breaks through the Veil slightly"weakens" it's local fabric, making it easier for other nemesis fiends to follow.
Given the result of both assessments, the Sentry then acts accordingly. If the caster is not ready to face his nemesis fiend yet, but there is still time, the training obviously continues. It is, once the time is about to run out, that things are becoming dangerous for the caster. If he is one of the lucky few who are allowed to take the final test, namely face their nemesis fiend and try to banish it, they have taken a huge step already. But not all, who approach the time of "transition", as the possession of a caster by his nemesis is called, are even close to being ready, .
Letting those who are not ready take the exam would just further the creation of another plaguefiend, which has to be put down by the sentry in a grueling and dangerous combat, and which on the other hand, simply by its presence, would weaken the barrier that protects other casters. Hence only those who are fit to do it, are chosen.
Those who are not chosen are turned into "husks", by a process called "curbing", some sort of a half physical, half magical lobotomy which severs the connection of a human to his personal fae, and thus also to his nemesis fiend, which is then left to wither in the counterrealm.
Husks are beings without ambition and ego, unable to feel joy or sadness, and must, for practical purposes, be considered almost mentally disabled. While their logic and reasoning remain intact, they are yet in need of care, because, left alone, they would just wither away too, unable to even care for food or the most basic needs due to their absolute lack of ambition, which includes the ambition to live.
But being chosen to take the exam does not automatically mean success, and that means the creation of a Plaguefiend, which the Sentry of a tower has to put down. But that means, that over time the veil becomes more and more permeable, which then again shortens the time casters would have to prepare for their test, making it less likely they succeed. The towers have thus begun to find other places to teach, usually about at least 30 miles away from the original "tower", and they change location to their winters quarter about once every year for about 3 months (usually beginning at Winters Dawn), so the veil in and around the Tower can recover. Then the whole tower moves its household, all books and people in a large procession which has become traditional part of the Winters Dawn festivities, where they are celebrated.

Bards

Bards have it worst of all. Their nemesis fiend is often highly charismatic, just as the bard, and the only thing which keeps a bard from being overtaken is that they understand just as well how to manipulate someone, as their fiend does, such that they come prepared. When a bard says he is fighting his inner demons... then this is more true than one might think.
However, the mental training that a bard goes through in the Bardic colleges, which are part of the towers, cut's him off from much inspiration, as if, somehow, the bard was connected to the rest of the world, and, by "suffering" the training, now loses this connection, as a price for longer intervals in which he doesn't have to face his nemesis, and easier face-offs with it.
Not every bard is willing to pay this price, which is a nice way to say that most bards suffer greatly, and, after a short time, rather flee than suffer any longer. The Sentry cannot let this happen of course, and keeps a keen eye on them, but even their ressources are limited. The task of a bard is thus, to find a balance between what he will take from his education, and what he won't, and when he considers the time come, that he has learned enough, weighing his creative development against the chance of being overtaken by his demons.
It may be a common cliché that most successful bards dropped out of school, but they did that for a reason. Those which stay and finally pass their test, may be sought after ministrels at court, yet they face difficulties creating someting new, for each new thing they create, could strengthen their nemesis fiend, something the have learned to prevent at all cost, and be it their own creativity. Being a mental block more than a real one though, over time, the bard can overcome this. It takes years though to finally find this sweet spot between creativity and stability, and many bards do not succeed.
Those who don't stay, if they do not succumb to their nemesis and thus become a case for the Sentry anyhow, often hide in wandering circuses or work as wandering musicians, always on guard about the Sentry and usually trying to avoid their attention. This eventually drives them towards an adventurers life, rather than that of a famous musician, although they might find their balance later in life nevertheless, and having banished their nemesis fiend quite often up till then (they are still alive yet...),they might even find acceptance... and fame.

Druids

Druids (and many shamans) have a different approach to magic, than wizards and sorcerors. Where to those it is a tool, the druid interprets the Fae as what it "is": The life force of Ardu. He tries to not disturb the flow too much, or at least keep a balance. For some reason this seems to work in their favor. Although many spells of the Nature Domain can be quite powerful, it is in their nature to keep and preserve, and they neither try to pollute the personal fae of a caster with the collective fae of the environment nor vice versa. As this is what creates a nemesis fiend in the first place, druids have a much longer time before their nemesis fiend "hatches" and "reaches adulthood". Where most arcane casters have two to five years after casting their first spell, before the time of transition nears, a druid can very well have ten or even twenty years, before they have to face their nemesis, usually as part of a cleansing ritual.

Origin/Ancestry

Fae Creature

Lifespan

2 to 5 Years. Limitless, once it overtakes it's caster.

Average Height

Same as Caster.

Average Weight

Same as Caster