Starting from depth 11, some monsters can receive one mutation, including boss monsters. The mutations are:
- Explosive: 0.5× HP, 0.5× defense. Monster is surrounded in an orange glow and explodes on death, much like explosive bloats. Will not appear on waterborne monsters.
- Infested: 0.5× HP, 0.5× defense, releases creeping death (lichen) upon death. Will not appear on monsters which already have an on-death effect.
- Agile: 2× move speed, 1.5× defense, flees when low on health, can blink. Will not appear on monsters which already blink or flee when low on health.
- Juggernaut: 3× HP, 0.5× move speed, 0.5× attack speed, 0.75× defense. Will not appear on monsters which keep their distance.
- Grappling: 1.5× HP, 0.5× defense, seizes prey on attack. Will not appear on monsters which keep their distance or already seize their prey.
- Vampiric: Has transference. Will not appear on monsters which keep their distance or already have transference.
- Toxic: 0.5× attack speed, simultaneously saps strength (like a centipede) and poisons (like a spider). Will not appear on monsters which keep their distance, already sap strength or already poison.
- Reflective: Reflects ½ spells; ¼ are reflected back to caster. Will not appear on spellcasters (as they are already hard to reach) and monsters which are already reflective.
Polymorphing a monster will remove its mutation.
Negating a mutated monster will remove any extra abilities added from the mutation. This includes the explosion on death from explosive, the creeping death from infested, the blinking from agile, the seizing from grappling, the transference from vampiric, both effects from toxic, and the reflection from reflective. The sidebar and the monster's description may still mention the mutation but these will be inaccurate.
The following table gives the chance for a monster generated on a given depth to be mutated. All possible mutations are equally likely.