Based off of this info, it's pretty apparent that Osiris, or at least his Cult, inhabited the Lighthouse. Plus, some of the flavor text on Osiris gear ties him/the Cult to Mercury, e.g. "You can't pull an all-nighter when the sun never sets."; "The sun took my sight, thus the sun became my eyes."; & "In the mirrorlike crags of the Fields of Glass, a single mote of dust produces infinite reflections."