Situated in the Deeprealm where Shigure was placed after his birth, the Astral Sea is a vast ocean that can be traveled by boat.
During Shigure's Paralogue, his Deeprealm was besieged by invisible forces forcing him to flee his home. Out on the Astral Sea, Azura, Corrin, and their servant are suddenly broadsided by the invisible forces on their way to Shigure's town. Shigure manages to make contact with them and the army drives off the forces. After the battle, Azura slips away momentarily to find Shigure's home and his caretakers all dead.