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.
Note: The dark aeons are only available on the 'International' version of FFX (the European version).
Dark Valefor
Preparation
Defeating Dark Valefor
If You DIDN'T Get Besaid's Destruction Sphere Treasure
Given Dark Valefor's position, you can't get back into Besaid until it's dead. This means if you missed the Destruction Sphere treasure in the temple, and wanted to use it to get Anima, then you are going to have to beat this horrifically hard boss first. At any level, this can be done, but it will take a while. First, you will need to ensure the following things:
- Someone in your party other than Yuna has a weapon with First Strike on it.
- You have Yojimbo (see the Cavern of the Stolen Fayth page if not).
- You have between 2,000,000 and 3,000,000 Gil (get a weapon with Gillionaire and fight Mimics in Omega Ruins if you don't). The more Gil, the better.
Dark Valefor HP: 800,000
Firstly, use Yojimbo a bit and make sure he uses lots of Wakizashis and doesn't die to raise his disposition towards you; 25 battles will suffice. Then, get him into Overdrive, but after that do not use any moves (Dismiss him or Grand Summon him if necessary). Create a party with any three characters, with one of them having the First Strike weapon. Now, go and confront Dark Valefor (save in the Promontory area with the Save Sphere).
On the first turn, switch that person out for Yuna and have her Summon/Grand Summon Yojimbo. Pay him every little bit of Gil you have and hope for Zanmato. This is entirely possible, but the chance is aroundabout 3% even by paying him upwards of 2 million Gil. The various things like increased disposition, Overdrive, and the option you chose when you got him will all have an effect on the Zanmato chance. If it isn't Zanmato (which it most likely won't, but it will always be Wakizashi otherwise) then do the handy soft reset with L1 + L2 + R1 + R2 + Start + Select, reload from the save, and try again. Eventually, you'll get lucky and Zanmato him, netting you a lot of nice stuff (hopefully) and access to Besaid once more.
This method does take a while but it is arguably quicker than levelling up your characters to the point where they would be able to confront Dark Valefor and win without Zanmato. (Note that if you are playing the EU version or you choose the option "to defeat the most powerful of fiends" when choosing Yojimbo it will be a lot easy to use Zanmato on Dark Valefor).
Dark Ifrit
Preparation
Defeating Dark Ifrit
Dark Ixion
Preparation
Defeating Dark Ixion
Dark Shiva
Preparation
Defeating Dark Shiva
Dark Bahamut
Preparation
Defeating Dark Bahamut
Dark Yojimbo
Preparation
Defeating Dark Yojimbo
Dark Anima
Preparation
Defeating Dark Anima
Dark Magus Sisters
Preparation
Defeating the Dark Magus Sisters
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).