- Fiend Soul
The user has a soul (or possibly souls) of various forms of monsters with different abilities. The soul of the user may undergo mutation in order to obtain this ability.
- May lose consciousness.
- May lose oneself to bestial instincts.
- Aki (Monster Soul)
- Elfman Strauss (Fairy Tail)
- Emil Castagnier (Tales of Symphonia: Dawn of A New World)
- Philuffy Aingram (Undefeated Bahamut Chronicle)
- Monsters (Undertale)