One of the neatest mechanics I've seen for this is in a Retro Clone OSR game "Crypts & Things" you have 3 degrees of magic that e veryone has access to (Anyone who has magic that is) White, Grey , and Black. Common sense can arrange them but when you dabble in the Black, you also lose sanity, sometimes permanently.
Just a fun mechanic and would keep those "darker side" players from becoming game breakers