Corruption

Corruption is a condition which distorts a person's magic and turns it against them. It occurs when a person is more sinful than virtuous. The sin that causes it depends on the type of magic; it's whichever sin opposes the virtue that powers the type of magic in question.
Corruption is specific to humans, as spirits' magic operates differently.

Symptoms

Corrupted magic will normally backfire. Though normal magic can backfire if used with sinful intent, corrupted magic will always backfire unless the caster has pure enough intent. Even with pure intent, it will cost the user. For example, corrupt life magic can be used to hurt or heal (with the right intent), but both come at the cost of the user's life energy. Even passive magical abilities are affected by corruption, so its effects can't be entirely avoided by not actively using magic.
In the most extreme form of corruption, called crisis, corrupted magic effectively "spills over" into another domain. For instance, transformation magic can't affect living beings, but if it's corrupted to its critical point, it connects to the user's life energy (connection to life magic). At this point, the user must overcome their envy, lest they become something horrific and inhuman.

Treatment

The only cure for corruption is overcoming the sin causing it and practicing virtue.

Prognosis

The "end result" of corruption is crisis. How crisis manifests depends on the type of magic, but every kind of crisis is horrifying. Thankfully, it's always possible to recover from corruption, even when it's become critical.
Recovering from a crisis is also known as taming it. Taming a crisis brings the "crossed-over" magic under the user's control—for example, enabling them to shapeshift voluntarily instead of involuntarily. Tamed crisis powers are similar to the transcendent powers gained by enlightened magic users (which also lie at the intersections of magic domains however, they come with the risk of becoming corrupt again should the user fall back into sin.

Prevention

Corruption can be prevented by living at least relatively virtuously.

History

As an innate part of the magic system and result of sin, corruption is inherent to the human condition (though many people never experience full-fledged corruption).
Its mythological origins are explained in two parts. First, a series of Golden Age narratives known as humanity's descent to corruption describe various groups of humans offending the gods and being cursed for it (such as the gluttons and drunkards at Lytheros' banquet, who he cursed to forever wander the mortal realm in search of pleasures as great as his). The Golden Age ends with all of humanity being cursed in one way or another and forbidden from returning to Paradise.
Later, in another tale, the goddess Techina, who created humanity, takes pity on the humans' plight. She conspires with another god (name TBD) to steal a portion of the gods' power and give it to humans in the form of magic. This angers Nomos, the king of the gods, but Techina persuades him to let the humans keep their newfound power—with two conditions. One, only the virtuous will be able to skillfully wield divine power; two, those who misuse it will have their curses amplified.
Thus, corruption exists to discourage sin.

Cultural Reception

Corruption is feared, and rightly so. That said, fear of corruption doesn't necessarily stop people from sinning. Some go to lengths to hide it, downplay its effects, or even embrace it as the "dark side" of magic. Not surprisingly, there is a lot of stigma around being corrupted, both because of the corrupted magic itself and the fact it results from sin.

Type

Magical