It's pretty clear that it's xur. Other than the obvious descriptions, the quote the "character" uses in the grimoire about the nine providing him purpose is a direct quote xur uses when he visits the tower in the vendor screen. For some reason, the nine wanted skolas released and their agent xur did it

It's either Xur or some other god-forsaken Agent of the Nine. I mean, who else has tendrils of darkness coming out from under a hood and says weird shit like that