Taint
A concept that shows up in a lot of fantasy media is the idea of taint, of pure evil literally staining the soul, if not the body and mind, and twisting the bearer into something warped and foul. It's rare, because it can get complex, but occasionally fantasy gaming tries to use mechanics to invoke the theme of taint.
Dungeons & Dragons traditionally uses the concept of forced alignment shifts to fill this role, and Ravenloft introduced the Powers Check mechanic. In 3.5, a late-publishing sourcebook called Heroes of Horror introduced Taint as a new game mechanic, acting as a sort of hit points for the soul and being more invested than the old-school alignment mechanics.
Pathfinder followed in the style of 3.5, with their sourcebook "Horror Adventures" basically combining Ravenloft's idea of "Terror Tracks" (Powers Checks that transform you into a specific kind of monster) with 3.5's Taint mechanics to allow a player to slowly become more and more of a monster.
Legend of the Five Rings has taint as the corrupting influence of Jingoku, warping body and mind to better serve the Dark Kami Fu-Leng.
Mists of Akuma uses a literal taint mechanic as a secondary ability score that can be increased by performing evil, being exposed to the attacks of certain monsters, or simply wandering around in the titular Mists, which are literally corruption manifested into a physical form. Taint can give you some powers, but mostly it'll just kill you and turn you into a monster if your level gets too high. Part of the appeal behind certain monstrous races, such as Onispawn, Bakemono and Shikome, is that you trade sucking at social interaction with regular humans for being absolutely immune to taint.