Mutation resistance

Version 0.12: This article may not be up to date for the latest stable release of Crawl.

There are several sources of mutation resistance in Crawl. Note that these do not stack; only the greatest source is considered.

• An amulet of resist mutation protects you from mutations 90% of the time. Mutations induced by magic contamination have a 50% chance to simply bypass it, while mutations induced by potions of gain foo are unaffected. Mutations from a potion of mutation will be blocked 90% of the time, but potions of cure mutation are unaffected.
• You may acquire the Mutation Resistance mutation. It has three ranks: at ranks 1 and 2, 66% of attempts to mutate you will be blocked. Additionally, at rank 2, 50% of attempts to remove mutations with a potion of cure mutation will fail. At rank 3, you can neither add nor remove any more mutations: they are fixed. The only way to cure them is to use Zin's one-time ability to cure all mutations. Halflings start with this mutation at level 1.
• Zin provides a (piety/200)% chance to protect against attempts to mutate you. Keeping his piety high enough to compete with an amulet of resist mutation is fairly expensive, but maxed out piety is a perfect defense against mutation attacks.
• Being undead makes it so that you will rot instead of mutate: in general, this is easier to cure than mutations. Ghouls and mummies will always rot instead of mutate, while vampires may do so depending on how full they are. Additionally, Necromutation causes you to become effectively undead, and so can be used as an emergency source of mutation resistance if you're prepared to deal with rotting.

Beyond that, your existing mutations also provide mutation resistance of a sort. Unlike the above examples, this resistance does stack, triggering only after a mutation bypasses your standard resistance.

• Each mutation you already have gives you 6.5% resistance against future random mutations. This ONLY applies to random mutations; good mutations gained through potions of beneficial mutation, or through use of the Helix card at high power, as well as bad mutations from enemy attacks, still affect you normally.
• Any time you resist a random mutation through this method, you have a 33% chance of losing one of your mutations as well. This allows you to use potions of mutation as limited potions of cure mutation if you find yourself suffering from many, many bad mutations, though it is by no means guaranteed not to simply make things worse.