After you defeat King Godwyn, Aquila asks you to free his teacher, a Celestrian who has been held captive in Gittingham Palace.
Oubliette - B1
Go east from the stairs and then south. After that, go west, passing through a couple of jail cells to continue. After the jail cells, go north. Follow the path and go down the stairs at the end.
Oubliette - B2
Go east into a hole in the wall. You will find chests that contain Battler's Bracers and a Mini-Medal. Go back through the hidden tunnel, then go to the southwest corner of this area and go down the stairs.
Oubliette - B3
Go north and check the bookshelves to get new recipes. Then continue going north and open all of jail cells and talk to all of the Celestrians. Then go to the northeast corner of the area and go down the stairs.
Oubliette - B4
Follow the straightforward path. When you reach the middle area, go east. At the end of the path, go down the stairs.
Oubliette - B5
Follow the winding path. When you reach a fork in the path, keep going north. Then when you reach an island with a hole in the middle, go west. Keep going and you will reach some stairs in the northeast corner. Go down the stairs.
Oubliette - B6
Follow the straightforward path here. When you reach a broken bridge, go to the east wall and go south across the boxes there. Keep following the path. When you see a room to the south, go into it, then go east to continue. Open the blue chest on the way. Keep going and you will eventually reach a jail cell. Open it, then walk toward the person in the middle. Talk to him. Answer his question however you want. When the battle begins, it doesn't matter what you choose to do, because none of your attacks will work. Afterward, watch the cutscene.
After the cutscene, you will be back in the Observatory. Go up to the Yggdrasil and you will get a Fygg. Then go into the Starflight Express and talk to Sterling. Then eat the Fygg. When you are ready, talk to Sterling and ask to go to the Realm of the Almighty, which becomes the Realm of the Mighty.