Remember the awesome looking Aeon which you can first witness at Luca with Seymour? Now this Aeon can be obtained as your own - or at least it can be obtained when you have the airship to freely use as your own.
All you'll need to do is locate Baaj Temple on the World Map. (X - 11~15, Y 57~63) When you get there, head back down in the water where you first encountered the monster at the beginning of the game. This time instead of running from it, you'll need to defeat Geosgaeno! This can be pretty tough since the only characters you can use for the battle are Tidus, Rikku, and Wakka, but with a little patience, you'll be able to take it down in no time.
After defeating it, you can enter the Chamber of the Fayth. When doing so you'll see six different statues lined up in the room. Now here is the part that can be pretty tough. Remember all the Cloister of Trials? Yeah? Well, you'll need a special item from each of those Cloister of Trials. Sadly, there is no way to go back to Bevelle's Cloister of Trials, so when you are there during that part of the game, make sure to completely explore the area and get the special item! Special items are usually weapons and special things like that. You'll need to insert them all into the statues.
The last item will be in Zanarkand Dome. You'll need to solve another puzzle which is displayed on the screen. After doing so another room will appear where you can ontain a destruction sphere. Grab it and place it in a slot near the moniter in the biggest room. Afterwards, you can collect the last item needed. Now head back to Baaj temple and get Anima!
Unique Attack - Pain This attack is one where Anima shoots a beam her eye, holding all the misey which it feels. When it hits the target, there is a small chance that it will cause instant death. Also, this attack usually does 9999 damage.
Overdrive - Oblivion A very long summon sequence which throws enemies into the depths of hell. Anima will cause large amounts of non-elemental damage to all enemies on the screen.