Corruption

Since It's inception, the Dark God knew that It could take advantage of the nature of Kin to show them physically what they have become: monsters. When a person commits a particularly evil act, they risk becoming Corrupted, resulting at first in a basic demonic transformation: horns, fire or raw energy burning from within, a cosmic, eldritch quirk, bestial or inorganic traits that shouldn't be there, and so on and so forth.
There are multiple stages of Corruption, all resulting from actions taken on a path of darkness.
Make no mistake, though: Corruption is something one does to oneself, not something that is done to them.
Demons take a particular pleasure in leading mortals on the path to Corruption, convincing them that what they are doing is right or absolutely necessary and slowly moving the goalpost until their prey realizes that they've gone too far and can't go back. They do this because of what happens when someone reaches the final stage of Corruption: total and irrevocable transformation into a full demon, bound to the Demon Realm and forsaken from escape to the Cycle.
Mages and any others who use a form of magery are no more or less inherently at risk of Corruption, but when they give in to dark emotions, their magic responds in kind. These acts of dark magic - or witchcraft, as they are also often called - are a very obvious sign to others that something is wrong.
There are multiple - but not many - ways of removing accumulated Corruption:

• One way to prevent reaching the next stage of Corruption is to wait; that’s right, just wait. Corruption goes down slowly over time as a person feels empathy and connects with others, and meditating can even aid this method. No amount of waiting, however, will fix a stage of Corruption you’ve already been afflicted with.
• Very rarely, one can take away a stage of Corruption on their own by managing to rise above their affliction for long enough to perform a good deed of major importance and lasting effect. This is difficult to do, since they are usually on a path that makes them hesitant to even try, combined with the fact that coming upon the opportunity to do such a thing is rare in and of itself.
• Powerful followers of some faiths are capable of exorcising the demonic influence from one's soul - usually followers of the Lights of Azariel - but this is usually a difficult to perform, and may require some negotiation with the person in question

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FAQ

How much does it take - generally - to reach the 4th and final stage of Corruption?
I'll use two example evil acts: murder and theft. I'll describe examples of how just doing one of those would each get you to progressive stages.
Murder is the most common cause of Corruption; vengeance, self-righteousness, etc. etc... they're all common causes for people to kill one another.

• Your first kill will Corrupt you and start you at Stage 1.
• You reach Stage 2 when you've managed to commit a small genocide; wiping out an entire town (or town's worth of people), killing a group of innocent creatures or creatures that are highly beneficial to the well-being of world, etc. etc.
• When you could be considered a mass-murderer in the most hyperbolic sense of the term, then you would reach Stage 3; at that point, you can't even count how many you've killed, and you probably don't care either
• To finally tip over into Stage 4 and become a full demon, you would then have to basically do what you did for Stage 3 twice over; essentially, when you've wiped out an entire nation's worth of people - actual genocide

Keep in mind, these are murders: the victims didn't entirely deserve to die, or they were killed very dishonorably, and so on and so forth. Just defending yourself or another person doesn't count - though there are always exceptions.
Theft is (obviously) a lesser crime all by itself, but there are people who will end up devoting their lives to it whether they like it or not.
It's generally harder to become Corrupted from thievery, unless you're stealing more important things like people's entire livelihoods, aspects of people's realities (through magic), people's love (through intentional, vile heartbreak), and more sentimental stuff like that.

• When you've stolen the equivalent of a person's entire livelihood, you become Corrupted and enter Stage 1; we're talking ruining someone by cleaning out their life savings, scamming people for years like you're a grunt in a Ponzi scheme, etc. etc.
• Stage 2 comes around when you've performed a certain number of true heists - planning, resource gathering, execution, escape, all of it; at this point, you're beginning to impact the economy of your local area and it could lead to problems for your surrounding community in the future
• Just like with Stage 3 from just murder, Stage 3 from just thievery means you've bagged so much coin and treasure across the realms that you've begun to forget some of even your greatest scores and replaced them with your most recent ones; this is where you're wiping out vaults from governments or large factions, generally putting an entire nation or entire nations into a small decline - which even that is a great feat for just one person
• To finally reach Stage 4 and become a full demon, you'd definitely have to hit one of those more intangible or at least impossible things to steal; leaving a god bereft of a holy artifact just because you wanted it and it was powerful, taking the lifelong research of an ancient Archmage on the cusp of an epiphany, and capturing beautiful pieces of the world to store in your own personal magical demiplane for your sole pleasure are all examples of this kind of thing

Type

Magical