So I watched the episode again on demand. During the beginning with Finn's nightmareafter they show Finn laughing at the tv it cuts to Billy and for a split second he is the lich. Got it to pause right at the moment and it was freaky, lol. Probably doesn't mean anything more than what happened at the end of the episode, but yeah.
And I don't really knwo what is supposed to be spoilered, so did it just to be safe.
I noticed the blip, but didn't catch what he was. Spooky.