Celestia is an upcoming world that is expected to be released in late summer 2010, although the "pre-quests" to gain access are expected in July. It will take place in an aquatic environment, with species appropriate to that.
A test realm for previewing the quests required to enter Celestia was released in late June 2010. Celestia is not, however, accessible yet.
Getting to Celestia requires players to complete a series of quests that involve trips to Ravenwood, Triton Avenue, The Oasis, the Tomb of Storms, the Village of Sorrow, and the Crucible. The final quest takes place in a new tower in Marleybone that includes several levels of powerful bosses of multiple schools, making it one of the hardest towers in the game -- the fact that the final boss cheats doesn't help matters. It is clearly designed to be completed by groups, not solo.
The picture on the right shows five new NPCs in Celestia; five more are covered in shadows.
Celestia will feature a new villain group known as the Shadow Weavers. For reasons not yet known, they do not wish anyone to get to Celestia. Of course, you learn this by being there, so their plans are already not working very well.
The June game update included updates to some NPC dialog to reflect the existence of Celestia. For example, in the quest Recover The Ring, Shelus GruffHeart now says, 'Hey there, esteemed wizard. I collect unique and rare relics from all over the Spiral. I was supposed to find a bloodstone ring for a young lady in Celestia, but the ghosts have it.'