Ixion is the third Aeon that Yuna can summon. She receives it after you complete the Djose Temple and Djose Cloister of Trials. Final Fantasy X is Ixion’s first appearance as a summon in a Final Fantasy game.

Yuna beginning the summoning sequence for Ixion
Ixion in the Calm Lands

Overdrive: Thor’s Hammer

Ixion can do regular attacks as well as an attack called Aerospark. In addition to doing regular damage, Aerospark can also remove helpful status effects from the enemy including Haste, Protect, Reflect, Regen and Shell. Thor’s Hammer, Ixion’s Overdrive ability, is a lightning-based attack, so be sure that the enemy you are fighting is not capable of resisting or absorbing lightning magic.

Ixion will be able to break the 9,999 damage limit after you have obtained Kimahri’s Celestial Weapon, Spirit Lance, and upgraded it using the Saturn Crest.