Stellarlune is a concept mentioned in Book 8: Legacy. Lady Gisela uses it on her son Keefe in 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.
Lady Gisela also mentioned that stellarlune also has something to do with potential, as she states that "all stellarlune does is use the natural forces in the world to bring out someone's full potential!"
Possibilities for what Stellarlune may revolve around the Latin roots used in the word; stella translates to "star" and lune translates to "moon." Shannon Messenger has used references like this before and it appears she has done it again.