The Basics
Aeons, call them summons if you want, are part of Final Fantasy X. There is a total of eight Aeons in the game and those Aeons can only be used by Yuna. Aeons are kind of a character in FFX, meaning that, when you summon them, they have turns like characters and you can choose commands, like any characters. They also have Overdrives, and at a time in the game, you will be able to make Aeons learn new Abilities and boost their HP, MP, Strength, etc.
Now were getting in the in-depth, each summons have special commands that you can access in any battles. Press right to show a set of commands. You have three command : Boost, Shield and Dismiss.
Use Boost to lower the Aeon's defense, thus you will take more damage from an enemy and your Overdrive gauge will charge faster. Boost will only last one turn, so don't worry about having to resist all the fiends attacks for the fight.
There is also the Shield option, select Shield to raise the Aeon's defense, it's pretty good when you know that an enemy will do a strong attack. Shield will block physical AND magical attacks, so it's pretty useful. The only disadvantage about Shield is that your Overdrive gauge won't charge. Same as Boost, the Shield effect will only last for one turn.
The last one is dismiss. Dismiss will make your Aeon go away and your party will return to the battlefield.
Now, once you acquire the Summonner's Soul Key Item, you will be able to make each Aeons learn new abilities. Look below for the list of every abilities possible to learn.
Ability List
Ability Items & Number of Items needed
Dark Attack Smoke Bomb X4
Sleep Attack Sleeping Powder X3
Silence Attack Silence Grenade X3
Silence Buster Silence Grenade X10
Sleep Buster Sleeping Powder X10
Delay Attack Silver Hourglass X20
Delay Buster Gold Hourglass X30
Zombie Attack Holy Water X99
Triple Foul Skill Sphere X4
Power Break Stamina Spring X8
Magic Break Mana Spring X4
Armor Break Lv.2 Key Sphere X2
Mental Break Shining Thorn X4
Pray Healing Water X5
Cheer Power Sphere X5
Aim Speed Sphere X5
Focus Mana Sphere X10
Reflex Speed Sphere X10
Luck Fortune Sphere X2
Jinx Fortune Sphere X2
Lancet Soul Spring X20
Doublecast Three Stars X5
Cure Hi-Potion X99
Cura X-Potion X30
Curaga Mega-Potion X60
NulBlaze Bomb Fragment X2
NulShock Electro Marble X2
NulTide Fish Scale X2
NulFrost Antartic Wind X2
Scan Ability Sphere X10
Life Elixir X6
Full-Life Megalixir X1
Haste Chocobo Feather X10
Hastega Chocobo Wing X16
Slow Silver Hourglass X4
Slowga Gold Hourglass X8
Shell Lunar Curtain X4
Protect Light Curtain X6
Reflect Star Curtain X3
Dispel Purifying Salt X3
Regen Healing Spring X50
Holy Blessed Gem X60
Fire Bomb Fragment X1
Thunder Electro Marble X1
Water Fish Scale X1
Blizzard Antartic Wind X1
Fira Bomb Core X2
Thundara Lightning Marble X2
Watera Dragon Scale X2
Blizzara Arctic Wind X2
Firaga Fire Gem X4
Thundaga Lightning Gem X4
Waterga Water Gem X4
Blizzaga Ice Gem X4
Bio Poison Fang X8
Demi Shadow Gem X4
Drain Stamina Spring X60
Osmose Mana Spring X10
Death Farplane Shadow X30
Flare Shining Gem X60
Ultima Supreme Gem X99
Dark Buster Smoke Bomb X12
Aeon List
VALEFOR :
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Yuna acquires Valefor at Besaid Temple at the beginning of the game. It's part of the story plot, so you can't miss it. Valefor is a flying Aeon, which means he have a high Accuracy rate. That means he is pretty good to dodge attacks and Valefor his pretty quick too.
Valefor's unique attack is Sonic Wings, it's not really strong, a regular attack hurts more, but it has a chance to delay a fiend's turn.
Valefor's Overdrive is Energy Ray. It's a powerful light beam that deals damage on all the enemies. You can also find a second Overdrive for Valefor.
Valefor's Second Overdrive : Once Yuna first joins your group, go back into Besaid and talk to person behind counter at the item shop (it doesn't matter if you buy anything or not). She will tell you something about her dog digging up something. Go talk to the dog in the hut closest to the temple on the right. The dog will give you something mangled and torn, which teaches Valefor the Energy Blast Overdrive. This Overdrive is almost twice as powerful as Energy Ray so it's a far more effective attack.
To make Valefor's Overdrive do more than 9999 HP damage, you will need to find Yuna's Ultimate Weapon, the Nirvana.
IFRIT :
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Ifrit is the second Aeon you get. You will need to get through the Cloister of Trials at Kilika Temple to get him. Ifrit is part of the story plot so you can't miss it. Ifrit is good for one thing, he starts with the Fire magic, so he can heal himself by casting Fire on himself. It will give you HP instead of damaging you.
Ifrit's Unique attack is Meteor Strike, it's a non-elemental attack that attacks only one fiend.
Ifrit's Overdrive is called Hellfire, it attacks every fiends on the field and it deals massive damage on every enemies. You can do more than 9999 HP of damage by finding Wakka's Legendary Weapon, the World Champion.
IXION :
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Ixion replaces Ramuh from previous FFs. You get Ixion after completing the Cloister of Trials at Djose Temple. He has a very high defense and you can use him as a bait for a fiend attack.
Ixion's unique attack is the Aerospark, which cause non-elemental damage on a single fiend. This attack his also like a Dispel, because it removes statuses like Haste, Protect and Shell.
Ixion's Overdrive is called the Thor's Hammer, it attacks every fiends with a Lighting elemental attack. It also attacks every enemies on the field and to break his 9999 HP of damage, you will need to find Kimahri's Spirit Lance.
SHIVA :
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You can acquire the Queen of Ice at Macalania Temple. She is a powerful Aeon that uses Ice Elemental for her attacks.
Shiva's unique attack his the Heavenly Strike, it attacks one enemy and delays the fiend turn. The attack also inflict physical damage on the fiend.
The Queen of Ice Overdrive is the Diamond Dust, it freezes every fiends on the field and Shiva breaks the Ice by clapping her fingers. You can deal more than 9999 HP of damage by finding Lulu's Legendary Weapon, the Onion Knight.
BAHAMUT :
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The last of the Aeon you can get in the story, Bahamut is an almighty dragon with powerful strength and defense. You can get him at the Bevelle Temple. Bahamut starts with powerful spells like Firaga, Wateraga, Thundaga and Blizzaga.
Bahamut's unique attack his called Impulse. It attacks every enemies on the field and deals great damage. The only disadvantage of that attack his that you lose a turn after you have used it.
Bahamut's Overdrive is the Mega Flare, which fires a destructive beam attack. It deals 9999 HP of damage and you don't need to find any items to make him break that limit. It will break itself on its own.
YOJIMBO :
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Yojimbo is a hidden Aeon, you can find him in a secret cavern after the Calm Lands. Just look in the Sidequests / Secrets section to find more about how to get him.
Yojimbo doesn't have any Overdrives attack, but it has three unique attacks, the Daigoro, the Kozuka, Wakizashi and the Zanmato. Yojimbo attacks fiends depending on how much you paid to get his services. You can make Yojimbo break his 9999 HP of damage barrier by finding Auron's Ultimate Weapon, the Masamune.
ANIMA :
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Anima is Seymour's Aeon, you actually fight Anima at one part of the game and there is a way to make Anima one of your Aeons. Return to Baaj Temple, you already have been there with Tidus at the beginning of the game, remember? This is where you can get Anima, the most destructive Aeon in the game.
Anima's unique attack is called Pain, it has a high chance of causing instant death on a single enemy.
Anima's Overdrive is called Oblivion, well maybe you already know that if you know what I mean :). Anyway, it's one hell of an Overdrive, since it attacks all the fiends on the field with multiple non-elemental attacks.
THE MAGUS SISTERS :
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Magus Sister right, remember FFIV ?! You can acquire this Aeon at Remiem Temple, but first, you need to capture all enemies at Mt. Gagazet and in the Calm Lands then return to the Monster Arena to win the Blossom Crown.
Then, acquire all the previous Aeons mentioned above. Then go to Remiem Temple located in the Calm Lands. You will fight Belgemine yet again, defeat her Bahamut to win the Flower Scepter. After move to the door in the back and use your two key items.
The names of the Magus Sisters' Special attacks are Razzia (Sandy), Camisade (Cindy), and Passado (Mindy). In Razzia, Sandy shoots daggers of fire at her opponent, in Camisade, Cindy uses a column of water to fire herself into the air and onto an enemy, in Passado, Mindy will shoot 15 stingers at a single enemy, this is the most powerful special attack of all Aeons because - if Yuna is properly leveled - each stinger will inflict up to 99,999 damage.
The Magus Sister's Overdrive is called the Delta Attack, the three sisters must be in Overdrive. You will see a new command called Combine Powers!. Then, the three sisters will unleash a powerful Overdrives on all the enemies.
