Summoning Aeons
After Yuna has entered a temple and received the blessing of the fayth, the aeon may be summoned in battle. The summoner must pass through a Cloister of Trails to reach the chamber of fayth. An aeon can packs devastating attacks, along with the ability to use magic, and the strength of their overdrive is simply amazing.

Other Aeon Commands
By pressing right on the directional pad when a aeon  is in battle, you can access a sub menu with more commands the aeon can perform. The commands are, shield, boost, and dismiss.

Shield causes the aeon to defend itself on the upcoming enemy attack. Therefore greatly reducing the amount of magic and physical damage taken on the next turn. However the aeons' overdrive gauge will not rise at all.

Boost causes the aeon to lower it's defense causing it to take more damage on the next attack(s). However the overdrive gauge fills twice as fast.

Dismiss sends the aeon away. The party will then come back and fight.

Aeon Abilities
Once you have received the item, "Summoner's Soul," you can use items to teach aeons new abilities such as stronger spells, white magic, black magic, and new abilities that the characters can also learn. You cannot teach abilities to Yojimbo, and The Magus Sisters. 

Aeon Attributes
After Yuna defeats Belgemine, she will receive the item, "Aeon's Soul." With this item you can increase aeons' attributes by using spheres. You can increase, HP, MP, Strength, Defense, Magic, Magic Defense, Agility, Luck, Evasion, and Accuracy. The number of spheres needed to raise one of the attributes depend on how high the attributes on the aeon already are.

[ Anima ]

Location ::  Bahj Ebon Temple
The dark Anima. First seen through Seymour. However the party can have a snatch at this one of a kind aeon through a side quest. Return to Baaj Temple from the airship, defeat Geosgaeno, and then enter the underwater temple. Find the six hidden items in the Cloister of Trails at each temple, and then approach each statue inside Anima's shrine. This opens the doorway to the most devastating aeon in the game, Anima!

Unique Attack :: Pain
Just look at the name, pain. The name really says it all. Anima conjures the essence of misery and anguish, and lets it all out upon a single enemy. The attack has a high chance of instant death. Pain has a long recovery time.

Status Attack :: Sleepel Attack/Silence Attack/Brain Attack, Sleepel Buster/Silence Buster/Brain Buster.

Black Magic :: Fire/Blizzard/Thunder, Fira/Blizzara/Watara/Thundara, Firaga/Blizzaga/Wataga/Thundaga, Bio/Death.

Overdrive :: Oblivion
In the most horrifying of all Aeons' Overdrives, Anima inflicts multiple non-elemental attacks on all enemies.

[ Bahamut ]

Location :: Palace of St. Bevelle
Bahamut possesses the auto-ability "Break Damage Barrier." This enable Bahamut to exceed the 9999 damage limit right from the beginning. Bahamut is a mad killer, with high Strength, Defense, and Magic attributes. Bahamut's regular attack is stronger than any other Aeons. Bahamut can also cast the highest level spells. Like I said, mad killer.

Unique Attack :: Impulse
Bahamut charges up and fires up to four balls of dark magic and hurls them, damaging all enemies. Impulse is very useful when fighting multiple enemies.

Black Magic :: Fire/Fira/Firaga, Thunder/Thundara/Thundaga, Blizzard/Blizzara/Blizzaga, Water/Watara/Wataga.

White Magic :: Libra

Overdrive :: Mega Flare
Bahamut anchors itself and fires a massive breath attack the rips apart everything around it. Bahamut's Overdrive is an extreme attack the constantly deals over 9999 HP damage and will continue to grow stronger.

[ Ifrit ]

Location :: Temple of Kilika
Though Ifrit has very high strength, it's defense and magic defense are low. Since Ifrit is an elemental creature (fire) he can heal himself with fire, fira, firaga. It is also best to use Ifrit against enemy's that are weak against fire.

Unique Attack :: Meteor Strike
Meteor Strike is a non-elemental attack that deal higher damage than his regular attack. It has a longer recovery time than his regular attack also. Meteor strike is a useful attack against long-range creatures. It also penetrates Protect despite it being a physical attack.

Black Magic :: Fire, Fira, Firaga

White Magic :: Ba-Fire, Ba-Cold

Overdrive :: Hellfire
Hellfire deals extreme fire damage on all enemies in with a fiery inferno of flames and rock
When Wakka gets his ultimate weapon, World Champion, all of Ifrit's attacks can exceed the 9999 damage limit.

[ Ixion ]

Location :: Temple of Joze
Ixion's high defense and high magic defense make it a good aeon to call out when the party needs protection from an enemy's attack.

Unique Attack :: Aero Spark
Ixions shoots razor-sharp discs at a single enemy causing non-elemental damage. The attack also negates magical effects such as, Protect, Shell, Reflect, Haste, Regen, and the Nul spells.

Black Magic :: Thunder, Thundara

White Magic :: Ba-Thunder, Ba-Water

Overdrive :: Thor Hammer
Ixion unleashes a server lightning attack, hitting all enemies with lightning elemental damage.
Ixion can inflict more than 9999 HP damage once Kimahri gets his ultimate weapon Sprit Lance.

[ The Magus Sisters ]

Location :: Ramia Temple
This trio can be yours by taking a side quest. Capture all the monsters in the Mt. Gagazet area, and return to the monster arena to get the Blossom Crown. Acquire all the other aeons, including Yojimbo and Anima, and return to Remiem Temple in the calm lands. Challenge and defeat Belgemine's versions of your aeons. When you beat her version of Bahamut she will give you the Flower Scepter. Continue to defeat Yojimbo and Anima. Then move to the door in the back and unseal it with these two key items.
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The Magus Sisters are like three separate aeons. As such, they are a bit difficult to control. There are verbal request that must be made, and weather or not they grant Yuna's wishes depends on the level of their morale.

Unique Attacks :: Camisade, Razzia, Passado
The sisters might perform their unique attacks randomly when the "Do what you will," command is selected, or they might do it when Yuna shouts, "Go, Go!" Cindy's Camisade shoots blades of fire at an enemy, while Sandy's Razzia launches her onto her target. Little Mindy's Passado will rapidly fire a burst of fifteen stingers at a single enemy.

Overdrive :: Delta Attack
When the magus sisters are all in overdrive mode at the same time, a new command, "Combine Powers!" appears. The sister will join forces and perform one of the most devastating overdrives in the game, Delta Attack. 

[ Shiva ]

Location :: Acquired in Macalania Temple
The beautiful goddess of Ice. An amazing sight to the eye and an agile attacker.

Unique Attack :: Heavenly Strike
Shiva drops a giant block of ice on the enemy slightly delay it's next turn and also causes some non-elemental damage.

Black Magic :: Blizzard, Blizzara

White Magic :: Ba-Fire, Ba-Cold

Overdrive :: Diamond Dust
Shiva freezes all enemies with sheets of ice, and destroys it with a gentle snap of her finger. She really lays down the laws. Shiva can break the 9999 damage limit when Lulu acquires her ultimate weapon, The Onion Knight.

[ Yojimbo ]

Location :: Cavern of the Stolen Fayth
Yojimbo is a secret aeon you can acquire by visiting the Cavern of the Stolen Fayth beneath the bridge at the northeast ends of the Calm Lands. To get Yojimbo you must fight your way through the cavern and beat Yojimbo in battle. Proceed through the cavern and speak to Yojimbo to negotiate a price for his services. He will offer his lowest initial price if you choose the option, "To defeated the most powerful of enemies." For your first bid, offer half of his asking price plus 1 Gil, so if he ask for 250,000 Gil, offer 125,001 Gil. After his next offer, raise your bid by 1 Gil. By the time he proposes 205,000 Gil, he will generally accept offers below 200,000 Gil, though not much lower. If at any point you offer him triple of his asking price he will give you 2 teleport spheres along with his services.

Unique Attacks :: Daigoro, Kozuka, Wakizashi, Zanmato
Yojimbo doesn't have selectable attacks or overdrives. As a sword-for-hire, he will attack enemies based on how much you paid him. Yojimbo's attacks will exceed the 9999 damage limit when Auron activates his ultimate weapon, Masamune.

[ Valefor ]

Location :: Besaid Temple
Valefor is very quick, and also very accurate. He easily dodges attacks from the ground.

Unique Attack :: Sonic Wings
Valefor's unique attack sends a shockwave of energy at one enemy, causing physical damage and delays the enemy's next turn. Because of the short recovery time, you can normally use Sonic Wings repeatedly on an enemy.

Black Magic :: Fire, Blizzard, Water, Thunder

Overdrive :: Energy Ray, Energy Blast
Valefor's Energy Ray fires a charged beam that explodes underneath all enemies causing non-elemental damage. Valefor also has a second overdrive that you can obtain when Yuna joins the party. Return to Besaid village, and speak to the little girl outside the temple. If she is not by the temple check out the item shop. She will give you an item that gives Valefor the overdrive, "Energy Blast." Energy Blast is similar to Energy Ray but it does more damage at the cost of a longer recovery time.
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Once Yuna activates her ultimate weapon, Nirvana, all of Valefor's attacks can break the 9999 damage limit.