An Oversoul occurs after a certain number of the same type of fiend have been killed. When Oversouled, the fiend's body absorbs pyreflies, acquiring a blue cast, and its HP and MP are restored and all status effects removed. Oversouled monsters have different items to drop and steal, and will yield more gil after battle. A successful steal can be done up to four times, as the items are reset when the monster Oversouls. Also, all abilities cost 0 MP when a fiend is Oversouled.
Thirty percent of enemies (54 out of 185) cannot Oversoul. No human-type enemies or the Doomstone, Imp, Larva, Revenant, and Spellspinner-type fiends can do so, but magically linked machina can. The classic monsters, Cactuar and Tonberry, can Oversoul, and Oversouled 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 this 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. When the fiend is Oversouled in the bestiary, Shinra jumps up and down. For Common Subjects, Shinra stands still. The Garment Grid The End with the ability Finale can be obtained from Shinra after every Oversoulable monster has been encountered in this state. It is unnecessary to defeat Oversouled enemies to add them to Shinra's list. Thus, if the player escapes from an Oversouled enemy, rather than kills it, they can keep the kill count to Oversoul other enemies of its genus as well. The "modified" enemies fought in the Fiend Arena in the International and HD Remaster versions do not count toward the 'Oversoul Subjects' list, however (enemies that use "unusual" abilities, like Quick Attack, Fireworks, Sparkler, etc.).
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 it 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. When an Oversoul encounter is due, and two enemies of that Oversoul type are encountered at once, 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 Elementals (level 9) are encountered together, and the one to gain Oversoul may be either the Yellow or the Red. 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. The specific species of enemy doesn't matter: if one kills 10 Haizhe and 3 Deep Haizhe during a hunt, the next Haizhe-type creature encountered will Oversoul. If one is trying to get an Oversouled Deep Haizhe and the 14th Haizhe killed is a regular Haizhe, one will have to wait to kill 13 more Haizhe-type creatures and hope the 14th will be a Deep Haizhe. That is perhaps why completing Oversoul lists on Via Infinito may be preferable, since the species of the same type rarely coincide on VI floors.
Requirements for each enemy type:
oversoul is a spiritual essence or vital force in the universe in which all souls participate and that therefore transcends individual consciousness. Oversoul can also be the universal mind or spirit that animates, motivates, and is the unifying principle of all living things. It's a concept in the transcendentalist philosophy of Emerson and others.In New England transcendentalism,
In the context of the Final Fantasy X series, the term may be rather literal, fiends gaining power when too many "souls" (pyreflies of their kind) have been released by exterminating the fiends.