It turns out that the vampire king had a son. And that son went on to become the king of vampires after marceline killed him. And now he's back for Marcy.
"You're not as mean as you think you are"
"And you're not as badass as you think you are"
"Oh, someone still has her panties in a twist"
"Don't for one minute think that you had any effect whatsoever on my panties"