Tainted magic is a house rule that is inspired by Dungeon Crawl Classic's and ACKS's take on the magic-user class. It is intended for Lamentations of the Flame Princess, where magic-users are already considered chaotic in alignment. The rules also affect elves, but to a lesser degree.
First off, the way new spells are learned is changed. The magic-user or elf doesn't really require a spell book. He gains a repertoire of spells. The repertoire for each level is equal to the number of spells the magic-user or elf can cast plus his INT modifier. Thus a 1st-level magic-user with an Intelligence of 13 would know two 1st-level spells.
When a magic-user or elf gains access to new spells, he can either choose among those spells he found during adventuring, or he rolls on the spell tables for the magic-user, or a combination of both. It takes one week per spell to add them to the repertoire.
To cast a spell, a magic-user has to roll a spellcheck (1d20 + Level + INT modifier) with a difficulty class of 10 + 2x spell level. If he succeeds on the roll, the spell takes effect and he retains the spell to be cast again. On a failure, the spell cannot be cast anymore before sleeping. Also, on a failure the spell doesn't take effect.
On a natural 20, any damage rolls are maximized, the caster may select any random results instead of rolling, and the saving throw of the target of the spell is penalized by the caster's level.
On a natural 1, the spell fails and cannot be cast again that day. Also, the caster must roll a saving throw versus magic. The roll is penalized by half of the spell level (rounded up). If the roll fails, the character gains a random mutation. Mutations are rolled on table Mutations (Carcossa, p. 256). If the character is a magic-user, he also ages by a number of years determined by rolling on the aging table on the same page in Carcossa. As an elf he is immune to ageing.
Overall, magic is available more often. But a magic-user will face certain dangers using magic and he'll also be not as versatile - more akin to a 3rd Edition sorcerer.