Well I didn't forget about this. If you've been working on getting all the fiends in your bestiary. You likely came across the set few for Ultima Weapon. Upon releasing Ultima Weapon who has a rather sad tale. You open up the Fiend Portal. Well this is the only Arena to carry a danger with it. If you die here it's game over. You need to make it to the end and defeat Omega Weapon. Omega is easy. Some of the fiends leading up to this fight aren't. Like Tonberry King.....Anyway survival is paramount. You don't recover HP between fights either. Well once this is over you get the choice of taking Ultima of Omega Weapon into the party. Might as well take Omega so you can get the bestiary completed and finish the story.