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The Dark Aeons (ヘレティック召喚獣, Heretikku Shōkanjū?, lit. Heretic Summon Beasts), are dark counterparts of each aeon in Final Fantasy X. Dark Aeons are one of the extra features included in the International, PAL, and HD Remaster versions of Final Fantasy X. Each has a red spot somewhere on their body, possibly an emblem to show that they are Dark Aeons, unlike their regular counterparts.
List of Dark Aeons
Once Yuna and her guardians escape from Bevelle, they are branded as traitors of Yevon and orders are given to stop their pilgrimage and kill them on sight. After defeating Yunalesca at Zanarkand and gaining control of the airship, the player is free to return to any location and look for new areas to visit.
The Dark Aeons now appear to halt the journey, some blocking access to previously friendly locations, while others merely wander in inconspicuous locations, hoping the player would come across their path. The first Dark Aeon the party may stumble upon is Dark Yojimbo, who can appear on the party's trip to the Cavern of the Stolen Fayth even before the airship is acquired.
Each Dark Aeon boasts a very high amount of HP, with Dark Anima taking the honors with eight million. They each have close to maximum stats and are immune to all status effects. All drop weapons and armor with Break Damage Limit and Break HP Limit along with other high-level auto-abilities, such as Ribbon.
Some of the Dark Aeons' abilities are non-elemental, such as Dark Shiva's Diamond Dust and Dark Ixion's Thundaja. Thus, Elemental-Eater abilities have no effect.
Dark Valefor
At the entrance to Besaid Village, a summoner disallows their entry and then summons Dark Valefor who must be defeated before the party can return to the village. The pre and post-battle cutscenes use recycled dialogue from early parts of the game. If Valefor's second Overdrive, Energy Blast, was not obtained before leaving Besaid Island for the first time, the player will have to defeat Dark Valefor to obtain it. Also, one of the Jecht Spheres Auron needs for his Overdrives is located in Besaid Village. Unless the player makes a long trek back to Besaid after Jecht Spheres are unlocked, defeating Dark Valefor is the only way for Auron to learn all of his Overdrives.
Dark Ifrit
Dark Ifrit is found at the entrance to the now-inaccessible Home on Bikanel. If the player approaches the former entrance, a summoner disguised as an Al Bhed woman will ask for their assistance, luring them into a trap where Dark Ifrit pounces if they try to peer over the edge of the canyon.
Dark Ixion
The first battle against Dark Ixion will initiate after talking to the Warrior Monk in the northern area of the Thunder Plains. Dark Ixion must be defeated twice before he will disappear. The second battle with Ixion can be avoided until the player is ready, allowing for items to be restocked if need be. There are differences between the first and second battle with Dark Ixion in the attacks he uses.
Dark Shiva
Dark Shiva blocks the entrance to Macalania Temple and is summoned by a group of Guado. Once she is defeated, the Guado will chase the party away from the temple, such as they did after the defeat of Seymour Guado. After the party escapes, it is once again possible to return to the temple.
Dark Bahamut
After the defeat of Yunalesca, Dark Bahamut takes her place in the Zanarkand Dome. If the Sun Crest was not obtained before embarking on the airship, the only way to get it is by defeating Dark Bahamut.
Dark Yojimbo
Returning to the back of the Cavern of the Stolen Fayth will cause Dark Yojimbo to appear and ambush the party. After his first defeat, the summoner will retreat back through the Cavern and the next battle will initiate upon their next encounter. Dark Yojimbo must be fought five times in a row for him to be truly defeated.
Returning to the save sphere to regenerate the party's HP and MP and to save the game will not reset Dark Yojimbo's battle count, however, if the player reloads their saved game or leaves the cavern, the battle count is reset and the player must start over from the first Dark Yojimbo fight. This can be used to the player's advantage by leaving the cavern after defeating Dark Yojimbo once, and then fighting him again. Repeating this method allows the player to obtain all the rare and powerful equipment they need to equip their party members.
Dark Anima
If the player returns to Mt. Gagazet, and completes Wakka's ball-throwing trial in the mountain cave once more, Dark Anima will appear at the entrance to the mountain. The party is not forced to fight Anima, so she can be spawned and then left alone until the player is ready.
Dark Magus Sisters
  
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- For their battle information, see: Dark Cindy, Dark Sandy and Dark Mindy.
The Dark Magus Sisters are found on the Mushroom Rock Road, near the exit to the Mi'ihen Highroad, and can be fought together or separately.
Once the Dark Magus Sisters are summoned, they will run towards the party, and if they catch them, a battle against all three sisters will initiate. If the player avoids them and runs towards the Valley, the Dark Magus Sisters will give chase. If the player then reaches halfway up the valley, Dark Cindy will give up the chase, so that if the remaining two sisters catch the party, Dark Cindy will not participate in the battle and therefore she will be fought separately.
If, however the player reaches the mushroom elevator at the north end of the valley, Dark Mindy will give up the chase, meaning the player will be able to face Dark Sandy on her own. Once Dark Sandy is defeated, the player must take the elevator back down to battle Dark Mindy, and then run back through the valley to fight Dark Cindy.
Fighting the three Dark Magus Sisters separately has many advantages, as the player can heal between battles, and the sisters will not be able to use their Delta Attack Overdrive. However, they will use Mega-Graviton as their Overdrive instead.
The Dark Magus Sisters can be evaded by reaching the save sphere, saving and soft resetting. Upon loading the game again, Mushroom Rock can be traversed through as normal unless the player runs into the summoners again.
Penance
Once the player defeats all Dark Aeons, Penance will appear as an option on the destination selection screen on the airship. Boasting 12,000,000 HP for the main body, and 500,000 HP for the arms, Penance is one of the toughest superbosses in Final Fantasy history.
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