Dark Anima is an optional boss available in the International and PAL versions of Final Fantasy X.
Redo the challenge in Mount Gagazet trials (the one where Wakka has to throw the ball into sphere). Then go to the save sphere at the entrance to Mount Gagazet (the airships drop location) and you will find Anima on the right hand side towering above the cliff. Go up to it to initiate battle.
 Recommended Battle Strategy
Anima has the highest HP out of the Aeons and is one of the hardest to beat because of the immensely high HP. By the time you even think of taking on Anima you must have the following: Ribbon Amour, Fully Customized Sphere grid with high luck and Maxed out everything else (around 60,000 HP and 999 MP). This is because at this stage the battles are nothing but hard and to do enough damage you will need to be prepared. Also you will need to have 99 Phoenix Downs and as many Mega Phoenix's as you can get and the Auto-Phoenix Ability. You will just need super speed (Auto-Haste it is a gods gift for this long fight) and high strength (this can be shortened by Wakka whose overdrive will do A LOT of damage). You will need relatively high HP as its Mega Graviton attack can actually kill you if you have low HP. It also uses death very often and unfortunately Ribbon doesn't protect against it that is why I recommended that you have Auto-Phoenix and Phoenix down. Use Life to save on phoenix downs from time to time as you will more than likely need to save them. When ever your given the chance use Wakka's overdrive it will make this fight shorter. Tidus's blitz ace can also do a lot of damage especially if his HP is high. So use them two for attacking. Their quick hits will also work wonders and take off 99,999 damage every turn. When Anima is about to use her overdrive quickly change to yuna and use a summon. You may need to sacrifice all of them in this long fight. But keep Yojimbo till last. Use him as your back-up option and if you have no idea how many more hits are needed to kill Dark Anima and it is about to use its overdrive use Yojimbo and hope for Zanmato. IF you think you've gotten it down to its last million, then I recommend you just use him to block, and then take it down to 0 yourself and get the self satisfaction. However, if you aren't too sure, its best to use Zanmato then spend 20-30 minutes fighting Dark Anima again in another attempt to defeat Dark Anima.
See Also: Dark Aeons