So, there's been some talk going around about the newest episode(Finn the human) and it's all true. I saw the preview for it on tv, and it seems to have the lich, finn and jake in the ep. The lich is warping through the multiverse, and finn and jake are chasing after him. It also has Simon Pertivok in it, watching birds (or planes) fly over him.
The Lich appears with 4 figures with glowing pink eyes behind him, seems to be taking over Ooo (which i belive is the start of the mushroom war, because ther is a pile of rubble, which seems to be the reamains of a city and this is basicly what Ooo looks like after the mushroom war)
Finn and Jake appear in their normal state and in the state in the end of The Lich(ep).From my thoughts, this is a truly awesome start to the season(better than the latest finale).