Due to the magical power located on the Summons' plane, time flows faster in the Feymarch. The town in which the Eidolons live is guarded by the Passage of the Eidolons, a cave filled with lava, and only those with the Float spell can bypass the lava and reach the Feymarch; crossing it without Float will cause HP damage. Rydia was brought to the Feymarch when Leviathan attacks a ship she is traveling on, and matures very quickly under the watch of the king Leviathan, after which she is sent out to help defeat Golbez.
After obtaining the Falcon, Cecil and his friends can travel to the Feymarch. There, they can interact with many of the land's denizens and fight Asura and Leviathan. After the Queen and King are bested, they will become available for Rydia to Summon.
In Final Fantasy IV: The After Years, in Rydia's Tale, the Mysterious Girl comes to the Feymarch to take the power of the Eidolons, Rydia was saved by the King of the Eidolons and so she could go to the rest of the Underworld to warn the Dwarves.
It is the structure farthest north of the streets. A nameless eidolon inhabits it, and the building also has the only Save Point in the whole Feymarch. To start the eidolon hunting game exclusive to the iOS version, the player must speak to the eidolon in the house, and when all eight are found, the player must talk to him to obtain the Piercing Magic augment.
The Eidolon Library is the building south of the eidolon's house. The first floor and B2 houses all tomes, scriptures, and books on Eidolons and Eidolon history, all of which the player can read as a summons crash-course.
It is the library's B3, accessible from B2. As the eidolon guard says, it is Queen Asura and King Leviathan's audience chamber. To speak to and challenge Leviathan, the party must defeat Asura first. After the two are bested, they will give the party their blessings and will wish them luck. During the ending, the audience chamber is shown again.
"Fey" is an alternate name of the mythical fairy. A march is the name given in medieval times for the border region of a country. The term was coined by the translator for the DS version, Tom Slattery.
“I also felt the creatures' world deserved a proper name. Who would call their own realm 'The Land of Summoned Monsters'? And hey, inventing words is always fun. 'Feymarch' had zero Google hits before the game came out; now it has tens of thousands.”
In the GBA and PSP remakes, it is possible for the player to visit the Feymarch without Rydia in the party, whereupon the residents will comment on her absence.
In the smartphone remake, the victory screen during the battle with Asura and Leviathan is the same as in the Passage of the Eidolons. In the DS remake, the Feymarch had its own victory screen. In the smartphone remake of The After Years, Feymarch has a uniquely adopted victory screen.