This is the story of how the Vampire King died. Pendelton Ward claims Marceline had killed him, but because I'm not an anti-Marceline fan, and I'm a very huge fan of her, I think she did not really killed him. Sure she's evil and nasty, but she's kind and won't even hurt an ant :). So I guess she just said she killed him. If she isn't the type to kill, she's that type to brag.
My theory starts now. This happens a thousand years before the show.

The vampire royals were out in the Grasslands, floating above the green grounds. The king had invited his lovely queen to a strolling in the said area earlier. Just the two of them.
Everything was peaceful. But it cannot last forever. As the two reached the vampire queen's tree fort, built by her and her former boyfriend Ash, they saw a shadow moving in the house.
"Look Marshall!" the vampire queen hissed, "someone's inside the fort!"
"Shut up, Marceline," her husband snapped, "I'll go check. Wait here."
He started to glide into the said tree house but Marceline grabbed his hand.
"Be careful." she said.
He nodded and opened the door. He sneaked in the basement and found a guy dressed in black clothing looting up the treasure fund in there.
"Hey you!" he shouted, getting the attention of the unexpected guest. Well, barely a guest, as guests do not just loot their host's gold.
"What?" he turned around, and the vampire caught sight of his face before the strange guy pulled his mask up.
"What do you think you're doing, looting up my wife's things!" Marshall Lee yelled and stood on the ground.
"You have a wife? I'm shocked," the stranger teased and snatched a sword from the pile. "Let's see if your wife can be proud of your swordsmanship!"
"You jackass!" Marshall charged at the guy, not even snatching a sword or a dagger.
"You fool!" the thief shouted, drawing his sword.

Outside, the vampire queen heard the screams and quickly dashed into the fort. Then she saw that the man was about to hit her beloved husband with sword. Without hesitation, she jumped in front of him and got hit by the sword. It cut across her stomach, and sent her bleeding. Then she fell onto the ground, knocked unconscious.

"MARCELINE!" the vampire king shouted, and dashed up to his wife's bleeding body. "You...saved...me..."
"Enough of your sucker romance! She's dying, you idiot!"

Of course, she wasn't dying. A vampire could not die because of a little toothpick like that, but a wound like that could take a couple of days to heal completely. Obviously the guy doesn't know who he's face-to-face with.

"NOBODY HURTS MY WIFE BUT ME! AND I DON'T DO THAT OFTEN!" The vampire king lunged at the guy, punched him then floated above him.
"You're a...a...vampire..." the guy said, getting up and sulking into a corner.
"Yes, I am!"
But that is when he made a mistake. He should've finished him off. In his range he failed to notice that a stake of wood was leaning on a wall nearby. The thief snatched the stake and drove it through the vampire's heart.
"AAAARRRGGGGHHHH!" He fell backwards and dropped near his wife.

"MARSHALL!" Marceline gasped, and crawled nearer to her husband, crying. "Don't leave me Marshall!"
He hugged her and said, "I'm not."

"That's quite enough of YOU!" the robber shouted, and kicked them both out of the house. They rolled down a nearby cliff and as they dropped, Marceline tried her best to pull them out with every bit of strength she still had left. But it was no use, she lost too much of her red-juice (or blood, those red they suck must go somewhere).

Marshall stroked her hair. "Don't cry," he said, wiping her tears.
"Marshall-" (cry) "-there's no use...I can't-" (more crying)
"Shhh. I may have only a few more seconds to live, but you could still survive,"

And with those last words he covered her and crashed onto the stones below.

a few months later

Marceline stared sadly at her destroyed home. Her fort was smashed and there were bis of wood everywhere. She turned her back on the ruined landscape and went away. Back to her home in the Nightosphere.

Along the way she met her friend, the Ghost Princess.
"Marceline, is it true?" she asked, "Was Marshall killed by a man?"
The vampire queen did her best to keep herself from punching the ghost, partly because it would be useless since her arm will just go through the lady's body and give her the cold feeling of nausea. She blinked back the tears.
"No," she said, trying her best to be proud, "I killed him. He was a total jackass."

And that's it. Sorry if it's short. I'm sad now D:.

Review and tell me what you think about it.