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 separate from the world of Fates. After choosing to abdicate herself from Anankos' control over her life, Lilith journeys to this realm, where she learns how to assume the form of an Astral Dragon to protect her sibling. This alone points towards the Astral Dragons' power of transforming any sentient being into a member of their species.
Astral Dragons also display the ability to travel between worlds. In Fates, Lilith is able to transport the Avatar to and from their castle's dimension and their world of origin.
According to the Fire Emblem Fates Visual Works Pellucid Crystal, Astral Dragons are a subtype of the First Dragons, said to be responsible for creating the world. Incapable of verbal speech, Astral Dragons communicate by means of telepathy instead. This is evidenced in-game, where Lilith's dialogue no longer has voice clips after she permanently assumes her dragon form.
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.