Body horror is a horror trope. It happens when a character is slowly turned into something different, or when another entity invades his body and turns it against him.
This can also apply to mutilation and other, less eldritch forms of modification - it's a significant theme in Cyberpunk for example - and applies, in a weirdly subverted form, to various mental disorders (generally called dysphoriae) where the sufferer comes to perceive their own body as alien, deformed or rebelling against them.
Body horror is distinguished from standard shape-shifting in that it typically happens gradually, is almost always unwanted (at least before the victim goes mad) or unexpected, and the victim is shown from a sympathetic point of view. An enemy werewolf is just another monster, but an old friend who's undergoing the change in stages is Body Horror.
Game and Story Use
- This can be an extremely frightening experience for a player character. Just make sure that the characters still have something to do until the conversion/takeover is complete. If the body horror effect chosen is so unpredictable and dangerous that the victim has to stay in a locked cell, then the victim can no longer participate in an adventure - which is bad for gaming.
- This can be rolled out when characters are subject to mutilation or transformation in the course of play - curses are also good for this.