It is off the CL Base camp. Go to the base camp and pull up the map, and it will show you. But it is one of those places you have to gain via main-quest line to be able to enter (without a friend helping).
It is in the Portico. Go to the Celestia Base Camp and up the ramp to the platform with portals to District of the Stars, the Portico, and the Chancel. Of course, you can't go in there until you get the quest. It's a dungeon.