Taboo

“People with weaknesses get killed by the people who lack them. Notice I'm not dead.”

— Sherrilyn Kenyon

A mortal creature can only get so strong, so tough, so fast or so magically attuned. There is a ceiling to any creature's ability, no matter what kind of creature it is. And sometimes that ceiling is simply too low, and to not go beyond means a fate that is unacceptable.
Going beyond that ceiling thus requires to alter a creature's nature, to make it greater than before, so that it may grow more powerful.
This is a deed not done lightly, as it is very likely to cause mutation of the body or soul, warping most that attempt it into monsters.

Those that relentlessly pursue power by any means necessary are almost always the worst kind of monsters. As a result changing one's nature on a fundamental level is Taboo.
Knowledge about the Taboo as well as how to break it is most often kept secret, and those that pursue this knowledge are most often met with open suspicion.
Scolarly institutions and the Clergy of Xaaris guard this knowledge jealously and factions like the Demons or the Malaci demand a high price for it.

Side-effects

Many that attempt to break the taboo fail and achieve only an early death, either because their chosen method was flawed or because some mistake or inadequacy on their part. This is a most inglorious end for an aspiring warrior or mage.

Even if they are successful, most methods of breaking the Taboo come with odds of signifficant side effects. Common side-effects include a shortened lifespan and mutation of the aspirant in body and/or spirit such as madness or physical illness.
Those that suffer obvious mutation are ususally hunted down by organizations vigilant for infiltration of society, such as The Raven Knights and Puritans of Samedi.

Still, even a partially successful ascention is judged worthwhile by many. A mutated superhuman is still a superhuman, and a killing spell is just as potent no matter who casts it.

It is notable that even a flawless ascendancy does not make the ascended immortal or immune to the ravages of time; they may live longer and be much harder to slay in battle, but they remain mortal.

Breaking the Taboo

The Taboo is caused fundamentally by mortal nature and the limitations of a physical being.
Thus, to break the Taboo is to change the fundamental nature of the person, most often by letting in something else, a foreign element to ascend the individual beyond mortal limitations..
This is not a decision made lightly, and never without consequences. It can cause irreparable damage to the individual's body, mind and soul.

Notable examples of breaking the taboo are:

1. Divine Baptism: Being uplifted by a deity is the most superior method of ascendancy bar none, however it is difficult to impress a god and this is mostly handed out in times of great need or to the innermost circles of the clergy.
2. Alchemy: Alchemical treatments to enhance a person to superhuman levels exist on a sliding scale of safety versus affordability. Fully safe alchemical methods are prohibitively expensive for anybody without the wealth of a kingdom backing them.
3. Undeath: becoming an undead, such as a Vampire is guaranteed to succeed but will twist the user into an unaging monster, likely in the service of another vampire.
4. Lycanthropy: Werewolves are magnificent creatures, though they have a tendency to lose themselves to their bloodlust, especially during a full-moon.
5. Dragon-blood: Bathing in and drinking dragon-blood is a possibility for those with access to it, such as Dragonslayers.
6. Demonic pacts: This is what demons are famous for; the highest bargain that they strike which usually costs the aspirant their soul.

Divinity

Those that have broken the taboo are able to accrue ever greater personal power. This can escalate to such an extent that the divines take note, and indeed they may judge the person worthy to become one of them. In such rare cases they may ascend the person to godhood, either immediately or upon their natural death.
Alternatively they may judge them to be too dangerous and direct their followers to hunt them down.