Final Fantasy X
Obtained Through A Summoner's Pilgrimage
In Final Fantasy X, Summons have been replaced by Aeons and they can only be used by a privilaged selection of Summoners. In this game, they do not require MP to summon and they can be summoned, or sent away whenever Yuna (the only Summoner available) decides to.
The ultimate purpose of a Summoner in Final Fantasy X, is to complete their pilgrimage and defeat Sin by using the Final Aeon, which can be obtained in Zanarkand. During the pilgrimage, they learn to summon sequentially stronger Summons in order to prepare them for the Final Summoning.
You Must Have FaythFor a Summoner to obtain a Summon, they must first complete the Cloister of Trials , which is a test of the Summoner and their Guardian's minds that appears in each of the Temples across Spira. Once complete, the Summoner is allowed to enter the Chamber of the Fayth.
Inside the Chamber of the Fayth, the Summoner speaks to the Fayth in order to obtain the ability to call a Summon. This can take hours, or even days, or they may be completely unsuccessful. Once the bond has been made, the Summoner can call on a Summon at any time.
A Figment Of Your ImaginationThe Aeons only exist through the power of dreams, the Fayth's dreams to be exact. This means that an Aeon is a physical realisation of the Fayth itself, as they convert their dreams into something of physical - rather than mental - form.
For this reason, The Aeons often have some kind of similarity to their Fayth and it also means that if the Fayth decides not to dream, the Summons will not be there for the Summoners to call forth.
More Than Just A One Trick PonyFinal Fantasy X offered players to actually control Summons for the first time. No longer were they just creatures that performed a single powerful attack for a set amount of MP.
Yuna could call forth the Aeons at will and they would then have their own HP, MP and Overdrive bars. To perform their ultimate attacks, they would have to obtain an Overdrive just like the normal characters. However, if they choose to Focus, which meant they took more damage, but increased their Overdrive bar more quickly.
If a Summon ran out of HP, then they could no longer be used by Yuna unless they were revived, which meant their usage was much more strategic.
The Final AeonAlthough all Summoners know they must ultimately summon the Final Aeon, which will take their life. Most Summoners who are undergoing their pilgrimage do not know that the Final Aeon will be one of their guardians.
When High Summoner Braska arrived at Yunalesca, Jecht offered to sacrifice his life so that Braska could defeat Sin. However, when Yuna arrives at the same place, Tidus convinces her that there must be another way and the cycle that had lasted for 1,000 years is broken.
Dark AeonsThe Dark Aeons are called forth once Yuna has been branded a traitor by Yevon. They're effectively Anti-Summoner Summons and are extremely powerful in comparison to the normal versions.
There is a Dark Aeon for each of the normal Aeons and once they have all been defeated, the player has the chance to fight against Penance. Penance offers the ultimate challenge in Final Fantasy X.
Final Fantasy X-2
Back From The Dead
Although the Fayths stopped dreaming at the end of Final Fantasy X, Aeons do appear in Final Fantasy X-2. Eight of the Aeons seen in Final Fantasy X return in their Dark form.
Unlike Final Fantasy X, the Aeons in Final Fantasy X-2 only appear as bosses and cannot be summoned as allies. They are nowhere near as strong as the Dark Aeons in Final Fantasy X though.
Manifestations Of HatredLike the Aeons in Final Fantasy X, the Dark Aeons that appear in Final Fantasy X-2 are also connected to the Fayth. However, this time they are manifestations of hatred, summoned by Shuyin.
His hatred for Spira is so great that it caused these Evil Aeons to be summoned forth, and they're found protecting areas of interest. For example, Bahamut was protecting Vegnagun in Bevelle.