Well the gems have power. Basically, the gems from the crowns, when put into the Enchirideon open a portal to another dimension. The Lich takes the form of the Hero Billy and tricks Finn and Jake into opening the portal. Finn and Jake follow the Lich into it and end up on a farm in what I assume is the American South where Finn has a robotic arm, his face resembles that of Lemongrab and his mom is there.
That is an amazing finale. They basically said the rules of connecting the dimensions and the reference that is master of it all is the one from "Five Short Graybles." The end was unexpected and a massive cliffhanger. Season 5 better begin explaining
Not at all what I expected but a solid 10/10