Farplane (Final Fantasy X)

In the world of Final Fantasy X, The Farplane 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.

Late in the game, the Venus Crest for Lulu's Celestial Weapon, Onion Knight can be found in a chest at the Farplane. During the events of the game, everyone in the party except for Rikku and Auron will enter the Farplane, Rikku refusing because the Al Bhed were taught to keep the living and the dead separated, and Auron refusing because he himself was an unsent, and felt he would be stuck in there. 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.

Final Fantasy X-2
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.

Abyssum