Oversoul (Final Fantasy X-2)

Oversoul is a condition in Final Fantasy X-2 where a fiend will spontaneously become stonger and gain more powerful attacks and abilities. An Oversoul occurs after a certain number of the same type of fiends have been killed. When entering the oversoul state, their body will glow and all HP and MP will be restored, and all status effects will be removed.

When in Oversoul, many fiends have better items available to steal, as well as more Gil. A successful steal can be done up to four times, as the items are reset when the monster is oversouled. The Garment Grid The End can only be obtained from Shinra when every oversoulable monster in the game has entered this state and been defeated by the party.

Not all enemies can be oversouled. Human enemies and Machina obviously can't, and the Imp and Ghost-type fiend do not either. However the classic monsters Cactuar and Tonberry can; in fact the Tonberry's HP will jump from 9999 to 39996 when he is oversouled! Tonberry also will only have access to it's Cry In The Night special move when in Oversoul and this is the only way for the Gun Mage to learn it as a Blue Bullet.