Things we know about the Mushroom War
1000 years ago there was a terrible war, and everyone was dying.
Marceline, a young half-demon, was left alone on earth.
Simon Petrikov, an antiquarian, was battling the evil of a cursed crown that had saved his life at a terrible cost. Simon met Marceline in the wasteland of earth, and they took care of each other.
The nuclear bombs came and “bathed the land in mutagenic horror”, but Simon survived because of his crown and Marceline survived either because she is a demon, or because Simon protected her.
Either way, the Lich was born out of man’s hate and desire to kill. Everything died or mutated, save for Marceline, Simon, and perhaps the few people lucky enough to get to underground bunkers.
After hundreds of years of death, mutation and suffering, the Land of Ooo came into existence.
However, that was only in one universe.
In the universe where Finn wished the Lich “never even ever existed,” the nuclear bomb did not hit earth. Instead, Simon stopped it with his mastery over Ice.
But in order to save the world from the bomb, Simon lost his life.
This fulfilled Finn’s wish about the Lich never coming into existence, because the bomb never went off.
However, the crown wasn’t happy with a dead master, but even it couldn’t save Simon from his fate. In a rage, the crown encased the world in ice for 400 “boring” years.
“Boring years” because there was no magic, no mutant creatures, and it looked like Marceline stayed by Simon’s side the entire time. She protected the crown so no mortal would ever get their hand on it again. Marceline never became a vampire, and instead, aged alongside Simon’s dead body.
Notice how there is no gaping crater in the earth that is usually shown in the Land of Ooo universe. Perhaps this means that it is the nuclear bomb that caused the crater?
When Finn puts on the crown and causes the bomb to explode, the Lich is once again born. In a world where the lich ‘never existed’ the Lich still comes into existence, perhaps showing the inevitability of the Lich’s presence. The Lich is the living embodiment of the consequence of war.
In both universes, Marceline is left alone.
And Simon is lost.