|Image derived from Wikipedia
I greatly favor the treatment of magic, chaos, and evil as all being different words for the same thing. Thus, protection from evil works on demons, elves, undead, and… sufficiently high level magic-users. Magic-users should become more inherently chaotic as they gain levels, until a sufficiently high level magic-user is essentially an alien being (despite having human form).
Previously, I came up with magical affinity
, which is good but insufficiently compatible with the traditional game (and maybe too complicated anyways). So here is another way to do it. The chance that a magic-user is chaotic for the purposes of a given situation is N in 6, where N is the magic-users spell competency (that is, the highest level of spells that the magic-user can prepare). I like this because the chance goes from minimal for low-level magic-users to certain for high level magic-users. Level / 2 round up would also work (or level / 2 round down, if you don’t want first level magic-users to have any chance of registering on the chaos meter).
This is better than an inverse saving throw versus magic (success = treat as chaotic), which is something I also considered, because it never gets to 100%. Further, an inverse saving throw is not straightforward to explain.
This is also the chance that a given magic-user will turn as a demon (of hit dice equivalent to the magic-user’s level) for clerics that have the power to turn demons.