Basic Information
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.
Corruption may also apply to areas as well as people - too much use of any kind of magic in a given area may mess with reality, and the worse the magic, the worse the effect of the pollution, up to turning the area into a bad place. Corrupt places may attract all sorts of unpleasant things, and potentially be self re-enforcing.
Sources
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.
- The Dresdenverse includes the wrinkle that part of magical corruption makes it easier to perform certain kinds of magic - although this may as much be a psychological process as a thaumatological one given the heavy emphasis on self-belief in working magic in that setting. The RPG offers concrete bonuses to each offence against "the laws of magic" after the first to model this.
- Your rules system may also have a parallel casting process which is easier (perhaps simulated by a bonus) or has greater supplies of energy, but which is inherently corrupt or otherwise inappropriate for general use. This may also be addictive and/or long term use may make it harder to do things properly - which may allow the bad guys a benefit from "casting black" but mean that, due to their persistent tendency to take shortcuts and grab easy energy, they are neither as skilled nor as powerful as someone who has always done things properly.
- Drawing power from an external source probably makes it harder and harder to voluntarily resist anything that power wants to do to you - this is an obvious problem for witches and priests, whose entire toolkit of supernatural powers is lent to them by their patron. A shaman has similar issues, but with less powerful entities. Even when the user is on good terms with their patron, they may find themselves being used against their will for some "higher purpose" on the grounds of implied consent … a malevolent patron, or one who has become annoyed at their client…
- This has the potential to serve as a metacurrency - for example, in Warhammer Fantasy Roleplay 4ED there are many ways for a character to gain chaotic taint … and for the GM to use that taint to influence the character's actions against the player's preference. Your taint might express itself as physical mutation, or mental disorder … or involuntary actions and sudden outbreaks of incompetence.
- Secondary magical corruption might also come from using evil weapons and other malign magical items - a particularly thematic version might be some kind of "power up" device that grants bonuses at the cost of a dose of corruption - examples might be the cannisters from the Geneforge games or the way the end of Days Gone left you wondering what exactly was in all of those "NERO injectors".
- A corruption system might apply to things other than magic - just being evil might be enough to earn points. This might only apply in a bad place or similar environment, or might be universal.
- Corrupting an area will be even more interesting mechanically… the Reality Warping option from GURPS Thaumatology might be a good place to start.
- There may be some flexibility in how corruption is earned, based on the character's attitude or similar soft factors - for example, an evil power is messing with the party and causes it to appear as though a cockroach wriggles out of one of the character's fortune cookie over dinner. One of the players has his character grab the roach and eat it. The GM then asks the player why his character did that: an answer suggesting that the character was showing defiance to the spirit might score only one point of corruption (since the PCs has still deliberately ingested a manifestation of evil), whilst one that did it to flex on his fellow party members or "hehe lol randomz" might earn, say, five points (and the latter player is probably in the wrong game unless they are deliberately portraying a character with serious mental issues).