Dark anima, the only one not listed above, is found by re-doing wakkas ball trial in mount G. you'll then see a cutscene showing you where he is (entranc to mount G) save before you attempt, he has 8 million hp, and an attack known as mega graviton, takes damge based on your max hp, he is the hardest dark aeon, defeat all the dark aeons, (i haven't but i do know this)and you get to fight penance, one of the hardest bosses in the game, possibly even THE hardest in the game