The Full Moon Crest is a quest item in The Elder Scrolls V: Dawnguard. It is one of three crests needed to operate the moon dial in Castle Volkihar's courtyard and open the secret entrance into its ruins.
Serana indicates her desire to locate her estranged mother, Valerica. She believes that she may have left a clue to her whereabouts somewhere in the courtyard of Castle Volkihar. The Dragonborn must travel with Serana and infiltrate the castle to discover the whereabouts of Valerica.
The Full Moon Crest can be found in the garden on the east side of Castle Volkihar's courtyard, between the nightshades planted by Valerica.