Forget what I said earlier. It looks like it's based around cursed flames, right? Now, I think it has something to do with the corruption. It's in a corrupt desert background, but I'm sure they could spawn it anywhere if they wanted to.
And another thing, Why are people assuming it's a boss? It doesn't look that big... Looks like part of an event to me.
(Hoping for a pre-hm event, nowhere near enough of those.)