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.
What if you save a vampire toddler and kept her for a while? Then her dad shows up and almost killed you thinking you kidnap her? But her dad is the vampire king who you never wanted to get involve with.
Dedicated to @shineeforeverr1412 and @daniellovespizza