Enthrallment

Enthrall or Mind Control is a recurring concept within the Final Fantasy series. It refers to the enslavement of a character, typically either a villain or playable character, by an evil power in order to do their bidding. With respect to the villains, it is typically not known to the player that they are actually puppets of another evil power until the story develops. The term does not refer to willing servants, but rather to an individual whose mind has been forfeited or has been otherwise strongly compelled to enter into servitude.

Final Fantasy IV
Multiple times in the story, Kain betrays the party in order to serve his new master, Golbez, whom after the incident at Mist Village had apparently placed him under a spell to enthrall him.

Golbez is revealed to have been enthralled all along, having been a pawn of Zemus in order to activate the Giant of Babil.

Final Fantasy V
Exdeath is able to possess people after the crystals sealing him have weakened enough; even if his physical body remains trapped, Exdeath's mind is able to control the King of Tycoon and Queen Karnak. Based on a Walse warrior's comments when a Garula attempted to shatter the Water Crystal about it being controlled, Exdeath was also implied to have possessed the Garula as well.

Final Fantasy VII
All Sephiroth Clones, including Cloud Strife, are subject to the will of Sephiroth and Jenova, especially with regard to the Reunion. Cloud is temporarily subjected to servitude to Sephiroth twice - once at the Temple of the Ancients, and again at the Promised Land, in both cases being made to hand over the Black Materia.

Final Fantasy VIII
In Disc 2, it becomes evident that Edea's malevolent actions are not wilful, nor do they comport with her kindly demeanor back when Squall and his friends were in her care as children. She reveals that as a sorceress, her powers and mind were subjected to the will of Ultimecia, who used Junction Machine Ellone to junction herself with sorceresses from the past. When aboard the Lunar Base, Rinoa comes out of a comatose state, only to be possessed by Ultimecia and free Adel from her tomb.

Later, as part of Laguna Loire's plan to assassinate Ultimecia, Adel and Rinoa are once again possessed by Ultimecia in order to accomplish Time Compression.

Final Fantasy X
Any new High Summoner must nominate a guardian to become the new Sin upon defeating the old one. Jecht, having volunteered to be the next Sin upon Braska's completion of the pilgrimage, is shown to be steadily losing touch with his humanity as he becomes Sin, a gargantuan beast who goes around destroying civilization under the command of Yu Yevon.

Final Fantasy X-2
Shuyin, a wandering spirit, is able to possess people and take over their minds. This is seen in Den of Woe where Shuyin possesses the Crimson Squad candidates to kill each other, and later tries to do the same to the Gullwings who enter the cave. Shuyin possesses Nooj, a Crimson Squad survivor, to shoot his friends, and later possesses Baralai and uses him as a host to control Vegnagun.

Lenne's spirit is also shown to take possession of Yuna's at times, although much lesser extent to what Shuyin is doing.

Final Fantasy XIII
While they are not mentally possessed, l'Cie, regular human beings, are enthralled by the fal'Cie, god-like creatures with great power, to do their bidding, on pain of turning into Cie'th - mindless beasts who roam about for eternity in torment.