Mysterious Entity after defeating Arbiter of Ash

I don't know if anyone has stumbled across it, but there seems to be a lil easter egg after defeating the Arbiter of Ash? My friends and I were beating the crap out of the Arbiter and after killing him, one of my friends saw an item drop on the ground. The camera suddenly panned over to the right side and this mysterious entity arrived on a dragon before saying, "This power serves a new master." I personally assume it has to do with one of the later acts, but not much is known at the moment. Has anyone else stumbled across this because I haven't seen much about it.