A mutation is a biological or magical change to a monster or more commonly to the PC. It might be purely cosmetic (like having changed eye color) or heavily affect gameplay. Mutations do not necessarily have to be bad; sometimes they are neutral for certain player classes or races or even quite beneficial.
There are many ways to mutate and it is specific for each game. Chaos presence, magical contamination, radiation, spells, items infused with distortional energy are just few to be named. In Alphaman, you are born with mutant abilities. Depending on the circumstances, the strength and type of change made to the subject entity may vary. Eating a corpse of a high chaos servant probably will twist the PC a lot more than indirect exposure to radioactivity would. Finally, some races have a tendency to mutate on level advancement.
A mutation may do none, some or all of the following:
- Modify a statistic, an attribute or other factors.
- Change, add, or remove limbs or other body parts.
- This may result in extra melee attacks.
- Give, modify or take away skills or special abilities.
- Change magical potential and connection to mana flow.
- Change the way shopkeepers, gods, and others react to you.
- Provide new attacks and defenses, or create vulnerabilities.
- Change your sex, race, class, or other "permanent" characteristics. See also polymorph.
- Make you wear tight spandex with X logos. No wait, that's just in the Marvel Universe.
Total quantity of mutation may also be important. For example, hunger rate might increase with each new mutation or a magical chaos gate might only let you pass if you have a minimum number of mutations. You might become more or less likely to mutate with increasing numbers of mutations.
Mutation is highly important in ADOM, where it provides a (limited) clock as mutation threat increases with game time and mutation is (unusually in a roguelike) potentially game-ending. Crawl and several Angband variants also have mutations, just among the major roguelikes.