Disclaimer: I don't anything from RWBY, X-Men, or Justice League. This isn't technically a crossover, RWBY and JNPR will just have powers similar to characters from X-Men and JL.
Where once stood a tall, proud tower, only remnants remained. Where once there was a bustling town, now stood only ruin and destruction. Fires blazed through the streets. Screams of grief and panic pierced the air. People ran, scrambling for any semblance of safety, only to be cut down by figures clad in white; their expressionless, silver masks staring without remorse at the bloodshed. Then, a flash of green, and a scream of pain.
Ruby could only watch, eyes wide and disbelieving, as Ren was slain in front of her. The emerald glow, the ring on his finger, fizzled and faded. Tears streaked her cheeks and filled her eyes, but her ears caught the cry of despair that ripped from Nora's throat. Then, a moment later, the sound of her hammer hitting the ground, and silence. Ruby didn't dare look at the sound of Nora hitting the ground as well. Her hammer, still sparking with electricity, slowly died.
Jaune and Pyrrha made a fantastic duo; sword and spear, twin shields battering at the enemy. It wasn't enough. Ruby saw the crimson blade cut through Pyrrha's neck like melted butter. She squeezed her eyes shut and sobbed, Jaune's cry of grief echoing Nora's as he launched himself at the enemy. A brief meeting of swords, then a choked gasp... silence again. Then, Ruby heard something that chilled her to the core. She heard the shout of her sister, and her teammates. Her eyes widened and she looked up.
Everything moved in slow motion. Weiss stood resolute, creating ice from nothing, streaking towards the man in black, trying to trap him. He deftly stepped to the side. Blake came next, locking Gambol Shroud with the crimson blade. Ruby knew Blake recognized the man in black. Her wide eyes and shocked gasp of "Adam" were enough for the now-named villain to pull his sword back and decapitate Blake as well. Ruby's heart nearly shattered, until she realized it was an illusion. Blake still had a nasty cut on her cheek, but she was alive.
Yang flew through the air- literally flew- her eyes blazing with heat and rage as she swung at Adam with all her might. Again, he side-stepped the attack. With speed only Ruby could see, he returned the blade to his scabbard. Some energy gathered there, before he released the blade and sliced clean through Yang's arm, right below the elbow. Ruby felt her breath leave her. Yang was supposed to be invincible! Nothing could touch her! And yet... here was clear evidence to the contrary.
The moment of shock was enough for Adam to strike again. His blade was swift, carving critical injuries into the chest of Weiss and Blake. Their eyes widened in shock, blood pooling from their new wounds, and they dropped to the ground. Ruby couldn't tell if they were dead, but it hardly mattered. Her heart was already broken. Somehow, the pieces shattered even further when Yang stood back up. She begged, pleaded, screamed in her head for her sister to just stay down! But her throat constricted, paralyzed with fear as Adam effortlessly grabbed Yang's neck and squeezed.
Yang choked and gasped, kicking her legs against Adam, trying in vain to loosen his grip. The fire left her eyes, tears started to gather, and the lavender orbs met Ruby's own silver eyes. With a rueful smile and what was likely her last breath, yang spoke three words:
"Run. Ruby. Run!"
A sickening crack, and the light left the loving eyes of her sister. Ruby felt a storm building inside her, then. A hurricane that promised to destroy everything in its path, so long as the end result saw Adam lying dead at her feet. Trembling, she pushed herself to stand. Adam glanced at her, looking almost bored; like killing everyone she cared about was nothing but a game. His blade scraped the ground, emitting sparks as he calmly strolled towards her. He pulled back his blade, preparing for the final blow, when Ruby finally let everything out.
She screamed, her cries reaching the heavens and beyond. Her pain, her guilt, her despair, every living creature in Vale heard it and raised their heads. Her silver eyes, still pooling with tears, stared defiantly back at the tyrant in front of her. He paused, only for a moment, his perception of her having changed. But then, he smirked, and raised his blade once more. As it streaked towards its intended target, the strike that would normally be a blur, slowed to nothing but a crawl for Ruby.
With herculean effort, she turned on her heel. Away from Adam. Away from the bodies of her friends. Away from the Tower of Beacon, now fallen. And she ran. Her feet pounded heavily against the stone street, her breath coming in ragged gasps. She passed the buildings without a thought, leaving the safety of the walls and running out into the Emerald Forest. There, trees stood tall, billowing in the breeze, yet frozen in place. Leaves fell, yet never reached the ground. Dark creatures, resembling wolves, bears, and boars were aplenty in this forest. None so much as flinched as Ruby ran past.
She didn't know where she was going, all she knew was that she was doing what Yang told her to do: Run. She emerged from the forest and came to the coast. She didn't even slow down as the edge of the water approached. She sprinted across the surface as though it were solid, disappearing beneath her feet and leaving a trail of mist behind her. Her eyes began to glow, silver sparks accompanying the streak of red that was Ruby Rose. She didn't notice. She just kept running.
An island came into view. Her home: Patch. Her feet hit dirt again, and she dashed through the trees towards her familiar home. She passed it, little more than a blur. Finally, she knew where she was going. She needed to see her mother. Faster and faster, she pushed herself, until her legs burned and her lungs begged for air. She saw the tombstone up ahead, the stone that marked her mother's grave, but she wouldn't get to see it. With another raw scream of emotion, the red and silver swirled around, gathering into a ball of energy. Then, in a flash of silver, she was gone.