Dark Ixion is an optional boss battle available in the International and PAL versions of Final Fantasy X.
Go to the Thunder Plains. Just below the save sphere in the middle of the thunderplains you'll find a man. Talk to him and he will run, run after him and he will attack along with a Dark Ixion. After winning this battle you need to play a sort of mini-game. When the lightning hits Dark Ixion will appear somewhere, when it hits again Dark Ixion will move. Get it the second time and beat it and you will have defeated Dark Ixion.
 Recommended Battle Strategy
The ability needed for Dark Ixion is Sleepproof as Dark Ixions normal attacks cause Sleep. Dark Ixion will use an attack called Thundaja however it cannot be blocked even with a Lightning Eater. Both fights are relatively easy. The first form is faster and will get more attacks in so auto phoenix is a must. Once its Overdrive is full it will cast Aero spark. Cast haste on the party as soon as you can and just keep attacking. Use all you overdrives on it if you need to. Once the battle is over you can save the game and then start the search for the second battle. The second form is very slow and so if you cast Hastega on your party you will get a lot of hits in (its best to use the recommended armor). Use hits that will definitely hit. Use Wakka's overdrive to make quick work of Dark Ixion's second form and beware of its overdrive as it will use Thor's Hammer this time.
See Also: Dark Aeons