Ever since he moved back into his cave after evicting Fionna and Cake from his old tree house, Marshall Lee the vampire kingkinda had a thing for Fionna, he couldn't quite explain what it was yet, but there was definitely something there.
That's why he let them move back into the tree house, it was the least he could do after all.
But he never told them of his ulterior motive, to befriend Fionna, maybe even progressing their relationship even further than that, but to his disappointment, despite giving the girls back "their" house, they stayed away from him, especially since Cake
was scared stiff of him.

He had tried to think of a way to get them to somehow hang out together, without Cake, but they were never too far apart!
So the only way he could do this was if he got her alone.
And he had a pretty good idea of how to do it...

Fionna and Cake froze in the middle of the path as they were walking home from Prince Gumball's palace at night when they heard someone sobbing.
They crept over to a bush, and peeked through,
Unbeknownst to them, Marshall had heard them coming from a mile away and had already set his plan in motion.
When he saw them peeking through the bush from his peripheral vision, he smirked.
This was going to be too easy!
Her self-proclaimed duty to save people was going to get her into trouble.

"You did it to yourself! Henchman for life MEANS HENCHMAN FOR LIFE!"

The child disguised as a little old ladyhe had paid to pretend to be his henchman started sobbing again.

He could hear the two of them whispering in the bushes.
"Dude! It's Marshall Lee the vampire king!" Fionna said in awe. Cake yelped and shrunk down to the size of a small bird, shivering and shaking like a leaf.
"You still afraid of vampires?" Fionna asked, and Cake cowered in the bush, flinching.
"N-n-n-no."
Marshall held back a laugh at the cat's reaction. He had to really sell this.
"Now cryfor me stupid bitch!" He said to the "little old lady"
The small figure looked up at him, hands shaking, and said,
" yes sir... I don't want to cry, but I will do it for you master!"
The person started sobbing again, and Fionna had seen enough, because she leaped out of the bushes, dragging Cake with her.
"HEY! Stop making that little old lady cry!" Fionna said angrily, then she bent down to try to help the "little old lady" up.
"Oh! Well if it isn't my favorite little goody-two-shoes Fionna!" Marshall said, sarcastically, " I can make this lady do whatever I want Fionna! Watch this!" Then the vampire king snapped his fingers, and said, "henchman, amuse me."
The little old lady tried to do a cartwheel, but ended up cartwheeling into a tree and hitting her head, crumbling to the ground, Marshall Lee laughed maliciously, and Fionna rushed to the little person's aid, she covered her with her body, shielding
her from further attacks, and Marshall sneered, as if she could stop me if I actually wanted to do something to her, he thought with amusement.
"Marshall I mean it! I-I-"
"Oh no! What are you gonna do hero? Have your stupid kitten boop me to death with her cotton ball paws? She's scared stiff you know!"
Fionna froze, he could tell she had no idea what to do, she knew very well he could easily kill her, and Cake too if he wanted to, and she thought and she thought, until she lifted her head up to the stars, closed her eyes in defeat, and said,
"Take me instead."
Perfect, he thought, exactly what I was going for...

"Done!" Marshall said, grabbing Fionna by her wrist and dragging her away from Cake and the person on the ground.
"Fi! You don't have to do this!" Cake cried,
"I HAVE TO CAKE! I'M BOUND BY MY WORD!" She cried as Marshall turned into a huge black bat, threw Fionna on his back, and zoomed away, laughing evilly, but then he toned it down a bit, feeling like he was overselling it a little.
"DON'T WORRY FIONNA! I'LL FIGURE OUT A WAY OUT OF THIS MESS!" Cake called after them, but they were already long gone.