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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 Sophie triggering it, although all information about this is unknown.