So i was doing a little research and then i found this :
S5 E1 Farmworld Finn and Jake in ditch

It seems its the other universe finn & jake

The last time we see this guys was being called by Finns Mother (Alt Universe) so maybe Finns mother need a help for something and then the Lich trapped them.Well this episode is still a mystery.

Well this is all my research. Maybe something will happen,Anyways its all up to you guys Make your theorys.