I think it's the Unknown. unless they are gonna add story quests for the place. I mean, we see the Shining Trapezohedron in Coils, which is a part of the story in Dragon's spine (mentioned by Ankh Ausar), it's a powerful "window" to other places and Dimensions give to the Serpent people by The Messenger.
Still find it odd that there isn't a continuation of the main DS story in Coils that will lead you back to Alanza or Ankh Ausar after discovering the Trapezohedron.