The Dark Margin, known as the Dark Shoreline or the Dark meridian, is a mysterious location first seen in the secret ending to Kingdom Hearts, Another side, Another story.... It also however is the location for 2 important character meetings. In all of the games it remains unvisitable, however Testuya Nomura has stated that the world is special as it lies on the "border" of the darkness.
Roxas and Xemnas
The events of this meeting take place during Day 6 of Roxas's time in the Organization. While the meeting was shown an overall 4 times (Another Side, Another Story...,Kingdom Hearts II, and 358/2 Days 2 times). Xemnas speaks to Roxas first. "I've been to see him...[Sora]...He looks alot like you [Roxas]." Roxas then asks him who he is, to which Xemnas replies "I'm all there is. Or maybe...I'm all there ever was." To this, Roxas replies, "I meant your name." Xemnas tells Roxas that his name is off no importance, and then asks if Roxas can remember his. Roxas can't. [In Kingdom Hearts II, a flashback of Sora's first memories are shown.] Xemnas then proceeds to stand up, and walk past Roxas. Xemnas mouths a word to him, which Roxas can't understand. Later on however, Roxas learns Xemnas actually said "Sora", which, is of course, Roxas' true name.
Aqua and Ansem the Wise
The "True Ending" of Kingdom Hearts Birth By Sleep takes place after the events of Kingdom Hearts II. Aqua comes across a hooded man in the Dark Margin, sitting in the exact place Xemnas was for his encounter with Roxas. Aqua meets the hooded Ansem. A hooded Ansem the Wise sits in the Dark Margin, wearing a Black Coat. Aqua appears next to him, asks his name and how he ended up in the Dark Realm. Ansem remarks how rarely he has guests and explains his memory has faded almost completely so he has difficulty recalling his past. Aqua sits next to him on the shore and states that she fell into the darkness and has been wandering for a long time, unable to find a way to her original world. Ansem asks why she wants to return, to which Aqua replies she must fulfill a promise made to a friend. Hearing the word "friend," Ansem remembers he had once known a boy who was very similar to her, a boy who is protecting the Light for his friends, the worlds and their denizens.
Ansem explains to Aqua that it appears as though the worlds have become swallowed by Darkness and are now being saved by a boy who wields the Keyblade. Hearing this, Aqua perks up and asks if the boy's name happens to be Terra or Ventus. Ansem fails to recall hearing those names and Aqua takes it to mean it's neither one. He explains further that it has been over a year since he had met the boy. He reflects aloud that through seeking vengeance he did many terrible things to the boy and his friends, causing so many people to suffer. He blames it on the fact he has the heart of a researcher, and reveals that as the boy slept, he hid his research data within him. Ansem realizes that the boy, who connects his heart to others, could possibly open the door and save those who have suffered, remarking that all is born from sleep. Aqua then requests the boy's name.