The Astral Dragon (星竜 Seiryū, lit. Star Dragon) is a Dragon class that is exclusive to Fire Emblem Fates. The unique class of Lilith, the only known Astral Dragon to appear in the game, the Astral Dragon class employs both its Breath and Staves as its equipment of choice.
Not much is known about the Astral Dragon species, apart from the fact that they reside in a realm that is disparate from that of the world of Fates. Lilith, following her choice to abdicate herself from Anankos' control over her life, journeys to this realm to learn how to assume the form of an Astral Dragon in order to protect her sibling. This alone points towards the Astral Dragons' power of transforming any sentient being into a member of their species.
The Astral Dragons also display the ability to travel between worlds, as evidenced by Lilith transporting the Avatar to the dimension that their castle exists in and back to the world from which they originate.
Seiryū is a play on the Japanese name for the "Azure Dragon of the East" (青竜 Seiryū lit. Blue Dragon), one of the Four Symbols in various Eastern mythologies.