Chapter 1: Corrupted

All Marco knew was pain. The Dark unicorn tossed its head aside, sending him splashing away to the ground. Marco writhed where he landed, his stomach swarming in agony from where the horn had impaled him. What should have been blood was a deep purple, the same color as Eclipsa's spells, staining his shirt and seeping through his body.

"Marco!" He heard rather than saw Tom skid next to him, kicking up golden flecks of magic. The demon made a horrified noise and retracted. "Dude-"

"I'm okay," Marco interrupted, carefully sitting up. "At- at least, I think I am." He stared at himself, bewildered. By all logic, that should have killed him. He supposed it was some sort of dark magic wound, like what the Solarian warrior's swords could give. He winced as Tom helped him stand. "We should find Star."

Tom opened his mouth but froze. "Okay, but maybe we should deal with that first."

Turning, Marco saw the corrupted unicorn rounding on them for another go. It charged, eyes piercing, nostrils flaring. Tom lowered his brows and got in a defensive stance. "Alright, Marco, on my mark… Marco?"

Marco felt the blood drain from his face. He crumbled to his knees and clutched his abdomen, seemingly random jolts of pain dominating his senses. What was happening to him? Vaguely he heard Tom say his name, but what was his name, anyway? And for that matter, who was Tom?

Marco screwed his eyes shut. Hold it together, Diaz. He couldn't lose his mind now, not when Star and all of Mewni were counting on him. Just think of Star. Think of Star…

Star...

The Dark unicorn was almost upon them, and a terribly agonizing pain pulsated through Marco's midriff. For a second he remembered everything, and he panicked to hold on, but as soon as his memories came they were gone.

"Marco? Marco!" Tom yelled, but Marco was far from hearing. His teeth clenched together, his hands balled into fists… then he stopped moving.

Tom froze, assuming the worst. Then Marco sat up. Impassively he looked at Tom, his eyes white and devoid of emotion. When he didn't say or do anything, Tom tentatively put a hand on his shoulder.

"Marco-?"

Marco looked at Tom's hand and his face hardened.

0o0o0o0

Star's plan was failing. Instead of the Whispering Spell destroying the magic, it only seemed to make dark magic stronger. The longer she, Moon, Eclipsa, and Meteora rehearsed their chant, the more vicious the creatures of the Magic Realm became. Hoards of corrupted Millhorses came out from the dark river, surrounding the Queens and Princesses, and no matter how many they wiped out, there was always a fresh wave to replace them.

"Glossaryck!" Star barked, casting spell after spell, "We could really use your help!"

Glossaryck could have been watching a mildly interesting thumb wrestle by the way he regarded the Butterflies and unicorns. "Hmm. The thing about that is- I don't fight."

Star growled frustratedly and punched a Millhorse in the face. "Nevermind- just go find Marco!"

"Ah- yes, Princess." Glossaryck spun around and blinked. In front of him was Marco, but he didn't look normal. His eyes were white- completely white, just like the Dark unicorn. Pearly whites showed in a furious grimace, his chest heaving and the royal wand clutched tightly in both hands. Marco's face glowed with crescent moons, a dark violet color, and Glossaryck hummed, studying him up and down.

"Well. That's not good."

"What now?" Expecting another dumb excuse, Star was startled when Glossaryck was thrown past, and instinctively she caught him. "Glossaryck, what-?"

Glossaryck groaned and pointed. Slowly Star looked, and her heart skipped a beat. Violet mist protruded from the royal wand and swirled through Marco's hair, emphasizing the darkness of his guise. Setting Glossaryck down, Star faced her boyfriend, eyes shimmering. "Marco?"

Marco acted like he didn't hear her. White eyes blazing, he raised the wand, sending violet plumes of magic straight for Star. Narrowly she avoided the onslaught of explosions and lost her balance. Marco aimed again, and this time Star wasn't able to dodge.

A second beam of light counteracted the dark magic. Star's eyes widened at the Queen of Darkness, Eclipsa's arms spread forward as she fought to quench Marco's spell. However the magic was too much and it knocked Eclipsa back, sending her sprawling in the golden liquid.

"Eclipsa!" Star scrambled to Eclipsa and rolled her over. She grimaced at the gash in her arm.

Eclipsa winced and put her free arm over the injured one, but smiled nonetheless. "Not to worry, dear- a simple scrape is all," She insisted, though her gritted teeth told Star otherwise. Her throat constricting, Star turned to Marco again. Tears welled in her eyes as he stared right through her.

"Marco…" she whispered.

Marco visibly shuddered. Something other than hostility flashed through him, but before Star had time to place it Marco broke eye contact and ran.

"Marco!" Star started after him, but was blocked by Firstborn.

"You must leave," the unicorn rasped. "Go back to Mewni- save your kingdom- there's nothing more you can do here."

Star skidded from side to side, but Firstborn continued to intercept her. "No- no, I can't go back to Mewni, not now, we're so close to ending the magic, we just need a little more time-"

"There's no time left! You must go, now! Mewni needs you!"

"Not without Marco!"

"Star!"

Star whirled at the call of her name, and nearly fell off her feet when Tom collided with her. As he realigned himself, Star sputtered, "Tom! Watch where you're going!"

"Sorry- it's just that we've gotta get out of here!" Tom yanked Star up and headed for the Mewni well. However Star wriggled free and rooted herself to the ground.

"What happened to Marco?" She demanded.

Tom responded in an urgent panic. "I don't know, but he's gone crazy! Look, it's not safe here, we have to leave while we still can!"

The world shook as the stomping of hooves drew near. The Dark unicorn neighed as it charged, and Star and Tom poised for combat. However something gold and white came between them, knocking the Dark unicorn off balance, and Firstborn reeled to face Star.

"If you don't leave-" She directed her horn, lighting it up. "-then I'll make you."

Star's reaction was immediate. Clamping Firstborn's snout shut, she snarled, "You are not separating us again!" Releasing her with a jerk, Star leapt over the unicorn and tore in the direction Marco disappeared to, then slowed to a halt. Bracingly she looked back at her family.

Moon and Meteora were fighting off the corrupted Millhorses in their Butterfly forms back-to-back. Eclipsa helped as best she could from the ground, only able to shoot magic with her uninjured hand. The suddenly pacifist Glossaryck simply evaded every attack. While they held their ground, it was clear they wouldn't be able to continue for much longer. Star didn't realize she had been crying when she felt Tom wipe away her tears.

"Look… the plan didn't work. Let's go back to Mewni, Star." He gripped her hand vice-like. "We can all face… whatever's going on there together."

Star shook her head, gasping. "I have to save Marco."

Firstborn raised her head, a fire in her eyes. "You leave me no choice." Her horn flashed, and after suffering a blinding ray of light everything went dark.

A/N: Hooi! I was watching the season 4 finale with a friend, and they guessed Marco's stab wound from the corrupted unicorn would turn him evil. I thought it was a pretty good opportunity for a fanfiction, and, well, here I am.

This chapter only took a couple hundred rewrites, so hopefully it turned out okay! What do you think? Should I continue?