Corruption, when considered with reference to magic, concerns the idea that prolonged used of magic - especially the less savoury kinds - leaves a physical mark on the worker's body as much as a spiritual one on their soul.
This can be used as an explanation for why wizards, especially evil ones, are always old, ugly and deformed: however they started out, extensive use of left-hand path magic has twisted them into something vile. Where it applies, it may well be a source and partial justification for memes such as ugly-equals-evil.
How it works is very setting dependent - in some cases, magic is a tool of Chaos and an inherently mutationary influence, in others magic is simply a tool that shapes the universe and corruption is the natural result of tool and user growing to fit one another - if your magic is ugly and unnatural, you will grow into an ugly and unnatural shape, just as your hand would callous around an oddly shaped hand-tool. Other explanations are possible.
For inspiration as to what exactly magic may do to a user, try some of the "tells" from glamour failure.
Game and Story Use
- Usually works best when built into the magic system - and is great when all magic corrupts, forcing PC users to weigh up a variety of costs when using.
- e.g. The magic system from The Riddle of Steel doesn't exactly corrupt, but does age users - meaning that heavy users of magic will always end up looking old, regardless how long they've actually been around for.