she turn into a vampire? huh? but isn't marceline the daughter of the vampire king?
She's half demon, so she ages slow but is not immortal. She became a vampire and usurped the vampire king's throne
I'm not so sure. I'm basing on the fact that Marceline aged in Finn's alternative reality. And I think the crown made the Ice Kind live as long as mareline would as a Vampire/Demon whatever she is 
I wish they would tell when exactly the war happened ;_;
I wish they would tell when exactly the war happened ;_;oh, i see.. Well, me too, for a fact that I also wanted to know how or what exactly happen at those times...