Star and Marco were sat at the dining room table one Saturday, doing some peaceful drawing.

Star was very enthusiastically colouring in a bright depiction of a Mewni picnic, while Marco was carefully sketching an impressive karate fight.

"Isn't it nice not to be doing anything for once?" Marco sighed happily.

"So relaxing," Star grinned, adding fire to her drawing with her orange crayon and almost indecent gusto.

They sat in happy silence for a while, enjoying each others company, and then Marco said,

"I'm just going to get my pencil sharpener."

When he returned, Star glanced at him, and saw something that caught her eye.

"Marco, what's that on your fingers?" She asked, pointing at the purple tinge creeping up Marco's right hand.

He looked down and jumped.

"What the..."

He stared at his hand in shock and confusion, and then suddenly, the purple began to spread.

"Star..?" He said, unsure, starting to panic. "What's going on?"

Star stared in horror as the purple spread over his entire arm. Marco took off his hoodie and his short sleeved T-shirt showed how fast it was creeping up his arm.

Suddenly, there was an almighty squishing noise, and the terrible tentacled monster arm flopped into existence and replaced Marco's right arm.

Marco and Star screamed.

"What's happening?" Star yelled, leaping up and dancing around hysterically. "Why is it back?"

"I don't know I don't know I don't know!!" Marco cried, shaking his monster arm frantically, as though it could be shooed off him.

But then he jerked violently and stopped struggling. He fell to the floor.

"Marco?!" Star shouted, using volume to mask her terror. "Marco, are you okay??" She dropped to her knees beside him on the floor. "Say something!"

Marco was breathing heavily and shaking, the monster arm pulsating and getting stronger.

He looked up at Star and whispered, his voice laced with fear and urgency,

"Star, you have to get away from me."

"What?" Star hissed. "Don't be stupid! I can help!"

"No, Star, listen to me-" Marco grunted. "It's taking over... I can't hold it off- it's going to-"

And suddenly, with the force of a bulldozer, Marco slogged Star across the jaw with his monster arm.

She reeled back and spat blood.

"Marco!" She screamed. "What the hell are you doing?!"

"I'm so sorry! I-" Marco started to cry, but then his eyes flickered and turned into a stranger's. Marco was momentarily lost as the monster wearing his skin advanced on Star.

Star knew Marco would never hurt her. Would never mean to hurt her. It was out of the question. Ludicrous. A ridiculous thought. A stupid fear.

A fear which was coming true.

She could see him fighting back: deep behind the eyes which belonged to someone else, Marco Diaz wore a horrified expression.

"No!" He screamed, actually grabbing the monster arm in his human one and wrenching it away from Star.

For a windswept heartbeat Star actually thought he had won.

Then he blinked.

And the whites of his eyes turned dark purple, and his pupils morphed into reptilian slits.

Marco was gone.

The purple spread further. Snaking over his chest- looping around his neck- trickling down his left arm- reaching round his back. It grew like a hideous disease and blossomed with tentacles and suckers. Marco's face was twisted in pain, his mouth open in a silent scream. Star dry-wretched as she watched her best friend being destroyed and replaced with this... monster.

Another monster arm quickly joined the first, and suddenly and horribly the purple was inching over the beautiful tan cheeks of Marco. Star sobbed aloud. She couldn't bare to see his face be lost.

She squeezed her eyes shut and turned away, and when she peeked round a moment later, Marco Diaz was dead. In his place stood a towering monster.

Star had no choice. It was the last thing she wanted to do, but she raised her wand and aimed it at the monster in Marco's shoes.

"Spider with a top hat," she whispered, her voice breaking.

The monster roared as Spider jumped merrily from the end of Star's wand and cocked his machine gun.

"Goodbye, Marco," she whispered, as the spider fired.