The Realm of the Banished is a world only accessible through the Realmwalker. Those taken into the world were never seen again.
The realm is a dungeon that can be found in Battlefields of Eternity in Diablo III. It can only be accessed after killing the Realmwalker, and is not guaranteed to appear. The portal is active while the Realmwalker is still alive, but only becomes accessible when players kill the beast: it is not possible to be drawn into it the way its previous victims were.
Inside, it is a common one-leveled Pandemonium dungeon, with exception that it contains a lot of unique monsters and trapped demons (and even angels) which were caught in the Realmwalker's portal long ago. There are also several portals in it, which, if players approach them, summon Champion Monsters of a random type picked from any act. An obelisk is located at the end of the dungeon.