Demonic corruption

Demonic corruption is the effect of demonic life energy on living things. It corrupts them poisons both body and mind. The most insidious is the mind's poisoning as it happens sooner than the body and is also far less obvious at first, slowly creeping in instead. It heightens what most people would consider bad personality traits, things like cleptomania, compulsibe lying, strong aggression and the like. its very common to develop sadistic traits, and what's worse is that once it gets far the person affected realises its happening but can't really do something about it, like they just don't realize it in the moment they are doing it, it takes an immense amount of willpower to even attempt to resist it. Once the corruption progresses further it also affects the physical body and soul. This will start to commonly give demonic natural magical abilities and grotesque bodily changes. This differs per person.
Inevitably if the affected creature stays in exposure to demonic corruption they will eventually lose their mind completely becoming a monster, while they might still have intelligence sometimes they are not themselves anymore instead being basically a full demon, only out to destroy that which is not demonic.