It seems like there wasn't a discussion for this yet (or I just couldn't find it) but here is one. The first trailer made me interested in what was going to happen, but then I saw the sneak peak...
....
It was kinda disturbing. I like it very much though, but yeah kinda disturbing. I probably is a dream, because we all know that Finn hasn't seen Billy after The Lich posessed him (until The Lich of course). But could there be a possibility that The Lich might come back? Feel free to discuss.