End of Sea

The End of Sea is a world in the Realm Between that reaches into the Realm of Darkness and was first mentioned in the Director's Secret Report XIII. The Dark Margin (闇の海岸) is located at the borderline of the two realms.

Kingdom Hearts 358/2 Days
On his sixth day with the Organization XIII, Roxas and Xemnas meet at the Dark Margin. Xemnas reveals that he has visited Sora at Hollow Bastion and comments on Sora and Roxas's similarities.

Kingdom Hearts II
Following Xemnas's defeat, Sora and Riku open a portal to the End of Sea, where they rest and discuss each other's friendships. Eventually, the duo encounter Kairi's Letter, which opens up the Door to Light and safe passage to Destiny Islands.

Blank Points
Aqua, after eleven years of wandering the Realm of Darkness, arrives at the End of Sea, where she encounters a hooded Ansem the Wise. Despite Ansem having forgotten his name, he reveals to Aqua that the worlds were in danger, until Sora saved them. Aqua sheds a tear upon hearing Sora's name, and says it with renewed hope.

Trivia

 * The world logo was first shown in the Kingdom Hearts Series Memorial Ultimania, but it has not appeared in any game yet.