A/N: I wrote this because I've been watching Supernatural season 15 and the whole Mark of Cain plotline is honestly one of my favorites.
This universe began with WBY, JNR, and Qrow standing in a bar. Ruby sat with her feet on a table across the room.
She sat up, setting Crescent Rose on the table in front of her. Ruby did not look amused.
"Hiya, girls." She said with mock enthusiasm.
Ruby snatched her weapon off the table as she stood.
"You don't have to do this." Jaune pleaded. "We have a cure." Ruby openly laughed at this.
"Wait." Nora said, "Is this evil Ruby?"
"I'm definitely getting that vibe." Qrow said.
"I don't think you're understanding that this damn thing doesn't come off!" Ruby laughed out, rolling up her sleeve and pointing at what looked like an F branded on her forearm.
"Is it a curse or something?" Jaune asked.
"We found a way." Weiss sternly said.
"I don't want it off."
"It's a lock." Ruby suddenly added from the back of the room. If she takes it off, an evil unlike anything you've ever seen will be released and destroy the entirety of the multiverse, including each of you."
"Where did it come from?" Weiss asked.
"I gave it to her." Ruby had a fond smile on her face, "I'm really happy with how it turned out. This is just the first draft, of course. It's not done yet, either, but this is the preferred outcome."
Silence filled the room for a second.
"It doesn't matter." Yang told her sister, "We can still fix this."
"Really?" Ruby teased, "Will we? Because right now, I'm doing everything I can not to come over there and rip your throat out. Leave. Last chance."
"Pass." Qrow simply said.
"I'm not walking out that door with you. I'm just not. So, what're you gonna do? You gonna kill me?"
"So this mark or whatever it is," Blake began, "It turns her evil?"
"It turns her into a demon, yes." Ruby explained.
"No."
"You don't know what I've done. Maybe I have it coming."
"I don't care. You're my niece, and I'm bringing you home."
Ruby raised an eyebrow, and laughed, "'You're my niece and I'm bringing you home'? What is this, a chick flick?" In one fluid motion, Ruby unfurled her scythe, swinging it at the man. Harbinger flew up and blocked the girl's weapon.
Weiss moved in on Ruby, sprinting past Qrow. Ruby swung the scythe back and clotheslined the heiress.
Yang sprung into action next, trying her hardest not to hurt the younger girl. She fired a few shotgun blasts that simply bounced off Ruby's aura. Ruby turned her scythe and fired one of its .50 caliber rounds at her sister. Yang was thrown through a table.
Jaune charged her next, bashing into her with his shield and pushing her back.
"Demon Ruby is kinda a badass." Nora commented.
Taiyang paused for a second before speaking up, "I want to disagree with you, but I can't find any good reason to."
"That's Nora for you." Jaune said.
"I really don't want to hurt you!" Jaune warned.
"Can't say I feel the same way." Ruby told him.
Ruby quickly rested her scythe on his shoulder and pulled the trigger. The blonde boy was roughly thrown to the ground, bouncing off the hardwood floor.
Gambol shroud suddenly flew at Ruby, wrapping around her legs and yanking her feet out from under her. Blake began to drag the girl across the floor and Nora ran at Ruby with Magnihild, the weapon raised above her head.
Ruby transformed her weapon into its gun form and fired it. The recoil pushed Ruby further away from Blake and pulled the faunas girl into the bullet. The large caliber bullet sent her flying through a piano.
"How will they play the dramatic music now?!" Nora cried, dramatically falling over.
Ruby grinned at her small victory, unwrapping her legs and transforming her weapon back into a scythe just in time to swing it at Nora's neck. The ginger woman went flying at Qrow, who rolled away to avoid getting hit.
Yang and Qrow both charged Ruby at once. Qrow choosing his standard Naruto-esque run, while Yang leapt into the air, fist poised behind her head.
"Time to get riggity-riggity-wrecked, sis!" Yang declared.
Ruby used her semblance to act quickly, swatting her sister out of the air and sending her flying into the bar before turning back to block Qrow's incoming attack. The older of the two swept Ruby's legs, but she activated her semblance, split herself in two and floated behind him to avoid crashing into the ground.
"Or not." Blake teased her partner at the apparent loss.
She swung Crescent Rose at his back, but he held up Harbinger to block it, spinning around to try and sweep her legs again. This time, she hooked her scythe around his calf and pulled her weapon's trigger. The man was sent flipping through the air.
She was poised to swat him out of the air when he came down, so he instead transformed into a crow, flying to the other side of the room rather than taking the hit.
Ren began to fire Stormflower at Ruby, running around the bar as he did so to make himself a harder target. He must have somehow forgotten what her semblance was or hoped she'd also forget, because she easily transformed into a red cloud of rose petals, splitting apart where he shot it, only to reform inches away from him.
"It's a trap…" Qrow simply said.
"How did you…?" Ren began.
"Because it's my job to know, kid."
She was swinging her scythe in a wide, deadly arc, but before it could make contact with the dark haired boy, Magnihild made contact with her stomach.
Ruby was sent flying across the room, smashing into the wall so hard that she bounced a few feet before slamming into the floor. She gritted her teeth and slowly pushed herself to her feet. They were all back on their feet now, standing in a line with their weapons drawn. Ruby snarled and closed her eyes. When she opened them, they were completely black.
"I'm gonna kill each and every last one of you, and I'm gonna love it!" She declared. The floorboard creaked to her right and she spun to face the newest challenger, only to get a flask of holy water dumped on her face. Steam rose from her face as she hissed in pain. Her aura fizzled and crackled a little but did not break.
"Dad?" Yang asked, shocked to see her father, and especially shocked at his next action.
"Time for some disciplinary action!" Weiss exclaimed, much to the shock of her friends and teammates. "What? I can't make jokes too?"
He cocked his fist back and absolutely rocked Ruby's jaw.
Taiyang cringed. "I pack a pretty mean punch. I hope she's okay." He said.
"DId you seriously just brag about bippity-bippity bopping your daughter?" Sun asked.
"He totally did." Yang confirmed.
"I did not brag!" Tai indignantly cried, "I simply just showed concern for my daughter getting clocked by a senior huntsman!"
"More like a senior citizen!" A voice called. Taiyang stood.
"Who said that?!"
Her aura broke at that last hit, and Taiyang pulled out one of those restraint things-the Ace-Ops had them in the latest episode of the actual show-and used to to restrain Ruby. She glared up at him. He sighed.
"I can't believe you actually hit her." Qrow said.
"Well, I didn't really have a choice."
"I'm not judging you, I just didn't think you'd do it."
"Let's hope this is the only time I have to." Taiyang knelt down and picked up his daughter's precious scythe. "This thing is filthy."
"It's just a scythe, Tai." Ruby matter of factly said.
"Feels real weird for her to use your name and not call you 'Dad'." Yang cringed.
"Well, she is a demon." Taiyang said, "They don't give a shit about familial bonds."
"They don't give much of a shit about anything." Ruby told him. "Except maybe killing and general debauchery."
"You really have gone dark."
A/N: Fight scene was paced to NF's WHY, if you were wondering where the chapter's title came from.