Astral Sea

The Astral Sea is a ocean located in a Deeprealms in Fire Emblem Fates.

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If Azura is married, she will have a child named Shigure, who is raises in this Deeprealm.

During Shigure's Prologue, Azura and the Avatar are sailing on a ship until Felicia informs Azura that a Pegasus is flying towards the ship. Much to Azura's surprise, it's Shigure. Shigure explains that he was being chased by invisible soldiers, later found out to be Vallite soldiers in Fire Emblem Fates Revelations.

After a fight against the attackers, Shigure sings a elegy to the people who died in the town he lived in within the Deeprealm. After a meaningful conversation, Shigure decides to join the Avatar's team.

It is unknown what lies beyond the Astral Sea, yet, as Shigure implies, there WAS a town nearby.