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.

Kingdom Hearts III
Soon after Aqua meets Ansem the Wise, Ansem, Seeker of Darkness, confronts the former king about the children used in his research. Though Aqua tries to fend off the Seeker, his guardian subdues her, imbues her with darkness, and throws her into the seas of the Dark Margin, transforming her into Anti-Aqua.

While looking for Aqua, King Mickey and Riku retrace her steps to the End of Sea. There, they are attacked by a Demon Tower, which damages Mickey's Kingdom Key D, snaps Riku's Way to the Dawn in half, and throws Riku into the seas of the Dark Margin. There, Riku encounters his replica's heart, convincing him to leave the Way to the Dawn at the End of Sea.

Later, after acquiring new Keyblades, Riku and Mickey return to the End of Sea, where they encounter another Demon Tower that restrains and disarms Mickey. Anti-Aqua emerges from the Demon Tower and explains the pain she has endured over the past decade, accusing Mickey for being "too late" to save her. With the strength of his replica, Riku battles the Demon Tower and destroys it with Sora's help. Sora then fights Anti-Aqua and successfully frees her from the darkness.

Trivia

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