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.

Note: The dark aeons are only available on the 'International' version of FFX (the European version).

Defeating Dark Valefor
With Anima (and possibly the Magus Sisters, if you want to make things really easy) in tow, you should be able to win this battle in just a few hits. Start the battle with Yuna (with a full overdrive), your strongest character (probably Auron), and your fastest character (Probably Tidus). It's advised that one character should have above 9,999 HP, as this is how much damage Energy Ray does.

If you've trained a decent amount, you should be able to get a turn with Yuna before Dark Valefor can do anything. If not, either cast Haste on her, or, if she happens to be killed in Valefor's first attack, revive her with another character. Any spare turns in between should be used for either Hastega, or getting as much damage on Valefor as possible before using Grand Summon.

On Yuna's first turn, Grand Summon Anima. Use Oblivion, and you should do at least 500,000 damage. (If you've been training, 700,000 is possible, and much better.) This will force Dark Valefor to have 4-6 turns to attack Anima before you can do anything about it. If it does not kill Anima, use Pain on Dark Valefor (it never misses, and will commonly do 99,999 damage). You could very easily Overkill Valefor at this point if all your attacks have done as much damage as possible until now.

If Anima died after the attacks from Valefor, either choose one of the following paths:
 * 1) Summon the Magus Sisters, and launch an all out offensive - their attacks should destroy Valefor quickly.
 * 2) Summon any other aeons you wish.  Yojimbo's attacks all do substantial damage at a high level.
 * 3) Use your full force with your strongest character - much more difficult than using aeons.

After the battle, you'll gain either a Master Sphere or Dark Matter (Two if you overkill), and a piece of armour that contains any of the below auto-abilities:
 * Break Damage Limit
 * Break HP Limit
 * Ribbon
 * Waterstrike

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).

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).