
That's what happens when you're only able to play the US version of the game.
The PAL release includes some optional super-bosses, the Dark Aeons, who're so ridiculously powerful they make Sin look like an utter sissy.
The Dark Aeons are shadowy versions of the regular Aeons, called up by Yevon's Warrior Monks. They're all incredibly powerful, dealing immense damage and sometimes status attacks that're unblockable except with Ribbon, an ability that's unique to the PAL version. Some of them have several million HP. The very weakest Dark Aeon, Dark Valefor, is extremely quick and had high evasion; every attack deals 9999 damage to one or all party members, and it can use overdrive abilities at will.
The other Dark Aeons are far, far worse. The Dark Magus Sisters can begin battle with Delta Hit, dealing about half a million points of damage to everyone. Dark Yojimbo's Zanmato instantly kills the entire party, even if they're using Auto-Life. Nasty stuff.
It gets even better, though... if you somehow defeat all the Dark Aeons, a new opponent is unlocked: Penance. It goes way beyond anything else in the game. Its very weakest attacks will do around 20,000 damage to a character with high defence, I believe.
It has 12,000,000 HP and three separate parts - the body and two arms. The arms can be revived by the body at any time, too.
Tough stuff. Adds quite a few more hours of replay value to the game...