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.

I just saw that also on my first arbiter kill. :) spooky

Ayee glad to see I'm not the only one who's seen it. It's prob a boss in the later acts, looking forward to what GGG will make.

When zoomed in while interacting with the object arbiter dropped - the flying creature actually has a bat head and it's rider looks a lot like a Lightless/Abyssal.

It actually looks a lot like the spirit that's present right before you fight the Vessel of Kulemak, which I'm sure is Kulemak in the Black Cathedral.

Being that the Arbiter of Ash fight takes place far underground - it would seem likely it was a Lightless/Abyssal, maybe even Kulemak himself, that came and took the orb that Arbiter dropped.