The TV Spot for Adventure Time gives away a lot in regards to what happened/what will happen. We now know that after going into the portal, Finn and Jake follow the Lich to a room that resembles the Time Room as shown in the map of the multiverse. A strange being that grants wishes grants the Lich's wish for "the extinction of all life." Aside from the question of why anyone would want all life to be eliminated especially when they could easily rule the world, Finn and Jake each get one wish. Finn wishes that the Lich never existed. While Finn is saying this, there are two scenes that are shown. One is the Lich moving through a ruined city with other similar sized undead beasts and the other is Simon Petrikov looking up at bomber planes flying overhead. It then cuts to Huck Finn waking up and saying "What?" in typical Finn fashion.
The clip can be found here: