- "The Farplane. The place that unites the living with their memory of the dead."
The Farplane (異界, Ikai) in the world of Final Fantasy X is a gathering place for the dead. They come here either after they have been sent or after they die (if they accepted death before they died). If the spirits are not sent to the Farplane, they remain as the Unsent and begin to envy the living, which then turns into hatred; thus they will eventually become fiends. The Farplane can be visited by the living, through an entrance at Guadosalam. The Pyreflies react to their memories to conjure an image of the dead person. However, if you think of a person still living, they will not be shown. The Farplane still is not completely understood by the inhabitants of Spira. The Guado are the keepers of the Farplane; as a result, they have the ability to sense the unsent.
During the events of the game, everyone in the party except for Rikku and Auron will enter the Farplane. Rikku, being an Al Bhed believes that the dead in the Farplane is just an illusion created by Pyreflies merely reacting to the memories of visitors (though she cannot explain how living people cannot appear). Auron refuses to enter for an unknown reason (perhaps thinking that, as he is an Unsent, he would not be able to physically leave upon entering). At this point, it is revealed to Tidus that his father Jecht is indeed truly alive, as he doesn't appear, while his mother does. While the group moves out of the Farplane area, they are met by the ghost of Maester Jyscal Guado. The visit of the spirit creates a great stir, since only spirits who died unclean deaths can leave the Farplane. Yuna sends Jyscal back into the Farplane. During the Sending, Auron falls on his knees, weakened as an Unsent would be by the ceremony.
Lulu visits the Farplane if the party returns to Guadosalam after the events in the Cavern of the Stolen Fayth to mourn the Lady Ginnem who remained unsent until the party defeated her. Tromell can also be found on the Farplane mourning Maester Jyscal and Maester Seymour Guado, but you only see Maester Jyscal, because Seymour stays an unsent until Yuna finally sends him inside Sin.
It is revealed late in Final Fantasy X-2 that the Farplane is actually at the heart of Spira, most likely meaning its core. Furthermore, the living can actually traverse through even the deepest reaches of it without the immediate danger of dying (as it is the location of the final battle of that game), unless one falls to the hordes of fiends residing there.
Two years after the Eternal Calm, Shuyin began his quest to destroy all of Spira and dug tunnels into each of the Yevon temples under the Fayth statue. These tunnels all lead into the real Farplane where Shuyin had taken Bevelle's ancient machina weapon, Vegnagun. Fiends began pouring directly into the temples from the Farplane; unfortunately, the Aeons which were once connected to the Fayth statues were also caught up in the wake of Shuyin's hatred and had to be destroyed by Yuna and her friends. The final battle against Vegnagun and Shuyin took place deep in the Farplane after Yuna, Rikku and Paine jumped into the tunnels to follow Nooj, Baralai, and Gippal.
After the party returns from The Farplane for the first time Shinra hypothesizes that there is boundless energy in The Farplane that could be harnessed to build cities full of light, like the Zanarkand of the past. However, Yuna soon realizes that this would never be accomplished within her lifetime.