Jake wasn’t there. Apparently Finn was caught unconscious. And death took on the face of Marceline And the world ended up being Marceline, just the traces of her face. It was a bit of a romantic story in the beginning. Like people had carved the world about her. So one day, they get a few of the people who were left in the land of Oo together. And she tries to help everyone achieve things. But Wildberry princess and ghost princess were the shyest. Marceline peered into their souls and was all BELEIVE. LET ME HELP YOU UNDERSTSAND THINGS. And then they went off to go do what was stopping them before. But apparently. Finn was like. In limbo. And Marceline was sending people off to make sure that the last path towards avoiding limbo and coming back to the real world was cut off. The bridges. The walls. Traps set. So every time she sent someone back to life, they had to make these immensely hard traps so that the person would fall and lose themselves forever in the traps. Ghost and Wildberry princess were on their way out, but Wildberry was hacking away at the ladder that finally would've let Finn wake up. She broke the ladder right after Finn got to her level. And tried to push him off the ledge (because it was either him or her to fall) And Finn turned it around. Apparently there was a flower that represented princess bubblegum's life somehow. And it was in danger of being ripped away by Wildberry princess too. And Finn had to push her off. I woke up when Finn came back to life. Like he managed his way out. But the world was run by death. Marceline wasn’t Marceline. But just death in her form. (sorry, kinda wrote it in a rush to get it all down)

TL;DR: Death took on Marceline's image and tried to keep Finn trapped in a netherworld. Finn awakens by having to sacrifice other princesses in limbo.