I don't own Demon Slayer or RWBY.
Blake Belladonna, in "The Final Word", she was dropped into the void under the Central Location by Cinder Fall, while trying to save Ruby from the same fate. Her current status is unknown. But that void actually lead to another world. Another dimension, filled with demons crawling everywhere in the night. She tried for 5 years to get back home, but nothing succeed. She lost faith that should would go home and learned to live with that fact she was never going home. Soon finding a Faunus Village filled with others like her, who were stuck in this world. Faunus were known for being demons for their animal like features and instincts in this world, but she didn't care. She had something close to home. And that was good enough for her. She fell in love, she got married, and she had a son. After 7 years of living in peace. Her village was attacked by Demon Slayers. Slaughtering the whole village to the ground, only her toddler, her a few others escaping thanks to the help of neighboring villages. But after two years off being on the run, she died while protecting her child. Her last words to him, driving him to the person who he is today. The second most powerful pillar, Kaigaku, The Shadow Pillar.
Also forgot to mention that everybody is in a different dimension. All them fell into different dimensions.
A request from ARCtrooper70.
The General plot is the following which may or may not be tweak:
Male Ruby Rose but in the Grimmverse AU. Starts with Grimm Attack on Beacon, separated from the others and is with Ozpin. Soon an explosion happens and knock out reader, injured and unconscious. Ozpin leave him behind, but his action didn't go unnoticed by the others plus Neo and Raven. Salem is good while Ozpin is evil. Salem offer WBY, Neo, and Raven an offer to free Remnant from Ozpin, by turning them into Grimm Human hybrids.
After two days Reader wakes up and see everything fell apart, his mission is to find his family and friends and team up with Adam. Later, find them and learn the truth and join them but not into the human hybrid, but special armor. (New Sith Troopers) and new weapon.
Together all of them join forces to defeat Ozpin once and for all.
When Ozpin was still Ozma, he want to conquer the world, and crush those that oppose him. Salem (before turning to Grimm) didn't agree to him and want to stop him. She tried to defeat him but failed, and the world went down to history to now. Since then she tries everything to bring hope into the world.
Adam is childhood friend; Neo is the same.
Beacon is Ozpin trying to make an army of huntsmen and huntresses that obey him. Teachers and Professors are involved. Ozpin doesn't care for the lives of students due to them being expandable.