Marceline is a half demon vampire born to Elise the human and Hudson Abadeer the king of the nightosphere around the 1990s to 2000s, right before the great mushroom war.
After the Mushroom bomb hit, creatures like Vampires begin come out to hunt the remaining humans causing a large decline in human population. Years later, after meeting and befriending a group of animal hat wearing humans, Marceline begins hunting vampires to protect the last remaining humans.
After practically exterminating all vampires in Ooo, including 4 out of 5 of “The Court”, the Vampire King attacks the human group as they’re about to sail away from Ooo. Marceline and him fight and Marceline ends up staking him but he’s able to bite and turn her right before he poofs, making Marceline the last and only vampire in all of Ooo.