|This spell causes aberrations to form in the caster's body, leaving the caster in a weakened state (though it was not fatal in and of itself). It may fail to affect those who are already heavily mutated.|
Alter Self was a level 7 Transmutations spell which gave you a random mutation, much like eating a fresh mutagenic chunk. This spell also damaged your health, but could not kill you directly. You could not cast this spell if your HP was too low.
Alternatively, distill a few mutation potions (gives more than 1 at a time), draw a 'Helix' card (better chance of a good mutation), or worship Jiyva who tends to give good mutations and a way to get rid of bad mutations.
Alter Self was removed in 0.9.