Gaf (a.k.a The Black Moon) is a Shadow Free Spirit which could be a single spirit or a group of similar spirits. Through a secret ritual, he creates a symbiotic link with each initiate of the Aleph Society, thus concluding a sort of pact that binds them to him.
Unlike other Shadow Free Spirits, Gaf is able to enter into more than one spirit pact at a time. As long as the initiates of the pact know the Shared Potency metamagic technique.
Gaf's true nature and motivations are unknown, it's hard to tell if it's a single entity, a group of spirits, or a whole new spirit type with unknown intentions.