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.

Breaking this 'Taboo' is seen as a great sacrilege by most mortal races; It is a deliberate violation of the design of the gods and is likely to bar one's entry into a variety of afterlives. It also has a very high likelyhood of harming the one attempting it or simply turning them into an insane monster. Mutations caused by breaking the taboo are likely to get you executed as a monster in most places, if found out.
Knowledge about the Taboo as well as how to break it is most often kept secret, as is a most heretical fact: After breaking the Taboo one's power can potentially keep growing forever, even allowing some chosen few to achieve Divinity.

Persecution

Those who break the Taboo are often persecuted, for the transformation turns most of them into monsters in some variant, if not in form then in spirit. Monsters that prey on others, consuming them to fill a hunger that has been awoken by a botched transformation.
To break with one's nature for power is an expression of such Hubris that it is unsuprising just how likely it is to fail.

In the past, most mortal races have witnessed multiple times some warlord or madman attempting to play god by turning themselves into a Monster. Some of these scars run deep and are not to be forgotten easily.

As a result, monster hunting organizations, witch-hunters and arbiters of law will all do their best to control the secrets of the taboo and hunt those that seek to break it.
Notable are The Raven Knights and Puritans of Samedi

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 let in something else, a foreign element to 'ascend' the individual beyond mere mortality.
This is not a decision made lightly, and never without consequences. It can cause irreparable damage to the individual's body, mind and soul.
Ascention is likely to result in Madness, physical deformity, health issues requiring medication, Mutation, unnatural hungers or simply death.

Below are listed some methods of ascension

Undeath

A very common way of breaking the Taboo is by becoming an intelligent Undead. This not only breaks the taboo but also grants the individual immortality, or at least immunity to the worst ravages of time. Examples include Vampirism.

However, with undeath comes a twisting of the mind, monstrous desires such as for blood or flesh of your fellow being as well as a disconnect with those one would have previously called 'friends'.

Lycanthropy

Lycanthropy is another way to breakig the taboo: infusing the body and spirit with so much life energy that they are able to take on a bestial, animalistic form. This grants longevity, vitality, health and strength.

Unfortnuately Lycanthropy lends itself to madness and savage blood-rages, as the curse of lycanhtropy is likely to overtake the rational mind to get the person to slaughter all around them if the afflicted is too mentally weak and guaranteed to do so irrespective of their strength of will if they are ever exposed to the full moon.

Alchemy

A relatively safe, if very expensive way of breaking the Taboo is using alchemical elixirs.
The recipes used to achieve this feat can vary wildly, as can the success chance, though even the most refined elixir has a great chance of complications.

Dragon's Blood

The Taboo can be broken by drinking or bathing in a dragon's blood. This can grant the ability to gain supernatural strength, speed and endurance, as well as boosting one's magical ability.

However this act merges the user's essence with that of a dragon. The user is likely to grow scales as well as wings and a tail, becoming physically somewhere between dragon and person, fitting neither. Their mind will be similarly affected, becoming apathetic to those not immediately important to them, becoming distrustful, greedy, arrogant and paranoid, unable to handle a dragon's natural predispositions.
Of course, to even attempt this method, one would first need to slay a dragon. As a result it is most common among the few bands of Dragonslayers that are willing to take the risk.

Fellstone

By imbibing Fellstone it is possible to break the Taboo. However this method is unusually deadly due to the sheer number of mutations and cancers caused by the substance as it comes into contact with living flesh.
The user is likely to end severely deformed as a result of this transformation, or, more likely, dead.
However this method of ascension is comparatively easy; all one needs is fellstone.

Divine Baptism

The Taboo is also broken if a mortal receives a Divine Baptism. This is a harsh trial, where the soul and body of a person is flooded with divine power. If they are unable to push through the pain, they die an inglorious death and are reduced to ash. If they survive, there is no risk of (undesired) mutation or mental impairment and their every attribute is strengthened by their new divine power.
However the only way to attain this method of ascension is to impress a deity enough for them to grant it as a reward. Seeing as this makes a mortal capable of being a threat to a deity and is very costly, it is only rarely done.
This is the only form of ascending beyond mortality that is not violating the designs of the gods, as it does not change the applicant's soul.