Final Fantasy X/Dark Aeons

Before you make any mistakes, be duly warned that the Dark Aeons are NOT for the faint of heart. They require an enormous amount of time and character optimisation with both equipment AND the Sphere Grid, and are incredibly difficult fights in their own right (unless you take the time to maximise your characters' potential beforehand).

The Dark Aeons are a group of very difficult superbosses which are the prerequisites to fighting Penance, the hardest battle the game has to offer. They are all essentially "Dark" versions of Yuna's aeons with the exception that they are all way more powerful and tough in comparison. Their stats are all extremely high, and they have very powerful attacks, and start out the battle with a full Overdrive.

Penance
In order for fighting Penance to even become available you must defeat all of the Dark Aeons. When you have, Penance will become a location somewhere above the Calm Lands that you can travel to using your Airship. Penance is one of the single most difficult and statistically powerful superbosses any RPG game-maker has ever devised, with an absolutely staggering 12 million HP and two forms (split into 3 million and 9 million respectively).