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This article contains plot details from the latest books.
Stellarlune is a concept mentioned in Legacy. Lady Gisela uses it on her son in the hopes of him manifesting a new ability, and the idea seems to revolve around shadowflux and quintessence in some way. Councillor Oralie is confident that it is related to the forgotten secret in her cache, and plans on triggering the secret with Sophie, although all information about this is unknown.
Possiblilities for what Stellarlune may be revolve around the latin roots used in the word; Stella transalates to star and Lune transalates to moon. Messenger has used references like this before and it appears she has done it again.