Oversoul is a condition in Final Fantasy X-2 where a fiend will spontaneously become stronger and gain more powerful attacks and abilities during battle. An Oversoul occurs after a certain number of the same type of fiends have been killed. When entering the Oversoul state, the fiend's body will acquire a blue cast and all of its HP and MP will be restored and all status effects will be removed. Oversouled versions of monsters have different items to drop and steal, and will drop more gil after battle. A successful steal can be done up to four times, as the items are reset when the monster Oversouls.
Thirty percent of enemies (54 out of 185) cannot Oversoul. No human-type enemies or the Imp and Revenant-type fiends can do so, but magically linked machina can. The classic monsters, Cactuar and Tonberry can Oversoul; and if it happens, the Tonberry's HP will jump from 9999 to 39,996. Mega Tonberry only has access to its Cry in the Night special move when in Oversoul, making Oversoul the only way for the Gun Mage to learn the Blue Bullet spell.
Shinra keeps track of the 'Oversoul Subjects' and 'Common Subjects' the Gullwings encounter, and Oversouls encountered are listed in red in Shinra's Bestiary; his enemy statistics, though, are only for regular enemies. The Garment Grid The End, with the ability Finale, can be obtained from Shinra after every Oversoulable monster in the game has been encountered in this state. It is not necessary to defeat Oversouled enemies to add them to Shinra's list.
Oversoul will always be the first action any monster of that type takes, and it will interrupt the timer of its next action to begin to Oversoul; however, it is itself an action and not instantaneous, and is possible to defeat enemies before they Oversoul. When an Oversoul encounter is due, and the party escapes from battle, the next encounter with that enemy type will still be an Oversoul. Encounter due, and two enemies of the Oversoul type are encountered, there is no level-based determination of which one will Oversoul; it appears to be random.
For example, in Via Infinito, Yellow (level 1) and Red (level 9) Elementals are encountered together, and on one occasion Yellow may Oversoul, or Red might instead. When such a dual encounter occurs, and the one that did not Oversoul is killed, and the party escapes from battle, the next will still Oversoul. This may or may not increase the count for the next Oversoul. Killing an Oversoul enemy itself may or may not count towards the required kills for the next Oversoul, and the count it adds may be greater than one.
The number of each enemy type killed determines when the next enemy will Oversoul. The numbers shown below are the number of kills required; the next such enemy faced will be Oversouled. E.g, the 21st Behemoth will Oversoul; running away does not count toward kills, nor does it reset the counter.
Requirements for each enemy type