Final Fantasy X
In Final Fantasy X, traditional summons appeared in the form of Aeons. As with Final Fantasy IX, their use was also restricted to a privilaged few, called Summoners. In this game, they do not require MP to summon and they can be summoned, or sent away, whenever Yuna or any other summoner decides decides.
The ultimate purpose of a Summoner in Final Fantasy X is to complete their pilgrimage and defeat Sin by using the Final Aeon. During the pilgrimage, they learn to summon sequentially stronger aeons in order to prepare them for the Final Summoning.
Entering The Chamber of the Fayth
For a Summoner to obtain an aeon, they must first complete the associated Cloister of Trials. These act as a test of the Summoner and their Guardians' minds and they appear 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 an aeon. This can take hours, or even days; they may also be completely unsuccessful. Once the bond has been made however, the Summoner can call on that aeon at any time to assist them.
Physical Manifestations of the Fayth
The aeons only exist through the power of dreams, the Fayths' dreams to be exact. This means that an Aeon is a physical manifestation of the Fayth itself, as they convert their dreams into something of physical form.
For this reason, The aeons often have some kind of similarity to their Fayth. It also means that should the Fayth decide not to dream any longer, the aeons will not be there for the Summoners to call forth.
What Happens When An Aeon Dies?
Final Fantasy X offered players the ability 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.
In battle, Yuna can call forth the aeons at will. They would then act as a party member, with their own HP, MP and Overdrive bars. To perform their ultimate attacks they would have to obtain an Overdrive just like normal characters would. To increase the rate at which their Overdrive bar increased, you could also choose to have the aeon "focus". This would see them take more damage as a result.
Should an aeon fall in battle, Yuna would no longer be able to call upon their services unless they were revive. This meant their usage was much more strategic than in previous games.
The Final Aeon
All Summoners know that when they reach the end of their pilgrmage, they must ultimately summon the Final Aeon at the cost of their own life. However, 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 therefore broken.
Why Did The Dark Aeons Appear?
In the International version of Final Fantasy X, the Dark Aeons are called forth once Yuna has been branded a traitor by Yevon. They're effectively Anti-Summoner aeons and are extremely powerful in comparison to the normal versions. Following the release of Final Fantasy X HD, all territories around the world now have access to the Dark Aeon story arch.
There is a Dark Aeon for each of the normal Aeons and it is possible to defeat them all. Once they have all been defeated, the player has the chance to fight against Penance, who offers the ultimate challenge in Final Fantasy X.
Final Fantasy X-2
Although the Fayths stopped dreaming at the end of Final Fantasy X, aeons do still 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 due to their weakened state.
Manifestations Of Hatred
Like 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.