If you read the grimoire the most compelling conclusion would be Osiris who was once the speaker's understudy. If you also take into account the rumor and theory that the next game, Destiny 2, is said to revolve around tracking down and finding Osiris, then this makes a ton of sense. also to entice this theory this very much means the possibility of the return and revamp of a Vault of Glass raid... on the down side as Osiris dabbled in time travel and vex tech this could mean that a time paradox(infinity loop) could be instilled into the story line of the game... which could mean soooo much re-skinning of the previous game... but also a chance the replay certain stories where you see things from an entirely mindbending point of view. just a theory but Osiris is said to be seen in multiple places at one time... look back at Kabr and the flavor text of his armor and grimoire and this makes a great example of what kind of paradoxical mindbending(VEXing) concepts bungie could very easily incorporate into the game. Shaxx is said to meet with a group of warlocks without shadows...all could be Osiris all at once...or another theory his followers(7 Seraphs) who could also be the ones responsible for reactivating Rasputin. again tons of speculation, but certainly fun to theorize and guess at.