Y/N is part of the demon slayer corp he uses wind breathing. He's also with Tanjiro, Nezuko, Zenitsu and inosuke. The group went with tengen to the entertainment district.
The group was winning against the two uppermoon demons. They were about to cut their heads when suddenly all of them heard a Biwa.
Then A woman in a Kimono appeared before them.
Gyutaro: M-master what are you doing here?
Muzan: I was gonna visit Daki but you guys just got attack.
Tanjiro: M-Muzan!
All the Demon slayers stood frozen when they heard who she was. Muzan looked at all of you guy but stopped and starred at Y/N. She disappeared.
Tanjiro: Y/N! Behind you
Y/N looked behind him to see Muzan standing there. Y/N was to scared to move a muscle, he was shaking profusely.
Muzan: So your name is Y/N is that correct.
Y/N swallowed the lump in his throat and nodded.
Muzan: I see well then goodnight.
Muzan knocked out Y/N.
Muzan used Telepath to talk to Nakime.
Muzan's telepath: Nakime I have someone i want to bring in the infinity castle. Teleport us to my bedroom.
Tanjiro: Y/N!
Tanjiro rushed Muzan and Y/N tengen was trying to stop tanjiro because Muzan was to strong.
Tanjiro: Hinokami Kagura 3rd form! Raging sun!
But before tanjiro can hit Muzan a biwa was heard again and. She suddenly disappeared and Y/N was nowhere to be found.
Infinity castle Muzan's room
Muzan: Ohh what a cutie i knew i made the right decision. now to wait for my darling to wake up we're gonna have lots of fun.
An hour later Y/N finally woke up. He looked around to see he was in an unfamiliar room.
Muzan: Ohh Darling your finally awake.
Y/N looked up to see Muzan staring at him with lustful eyes. He noticed he was tied up in the bedpost and without clothes.
(Y/N): where am i! Why are you doing this!
Muzan: ohh Y/N it's wrong to yell at your wife like that. You see I Love You Y/N it was love at first sight.
(Y/N): And you think I'll love someone like you.
Muzan: Okay The hard way it is.
Muzan cuts her finger and blood was pouring out. Muzan then grabbed Y/N chin and forcibly opening his mouth. The blood dropped at Y/N's mouth instantly Y/N started thrashing around. After twenty minutes of thrashing it finally stops and Y/N started to wake up again.
(Y/N): W-what happened?
Muzan: It simple darling i turned you to a demon.
Y/N was shocked and looked at his appearances. He saw he was paler and longer nails. He checked his mouth and felt a long and sharp tooth it was fangs. He couldn't believe it he was actually a demon.
Muzan: Do understand dear this was the only way for you to accept my love.
(Y/N): Human or Demon i will never accept your love.
Muzan: Okay then I'll force you then.
Muzan strips her clothes of and also removed Y/N's after that Muzan raped Y/N over and over again. After that he still wouldn't accept Muzan's love.
Muzan finally used her trump card. Her mind altering technique. Which she learned a while back ago. She positioned one of her finger through Y/N's head. Then it pierced right through his skull and in the brains.
Muzan altered Y/N's mind. Muzan erased Every memory of Y/N and his past and created fake memories. Where Y/N and Muzan lived happily as demons.
Y/N woke up and greeted his wife.
(Y/N): Ohh Honey How are you today?
Muzan: Yeah I'm fine darling. Look it's Night we should go eat.
(Y/N): Yeah your right let's go.
Muzan's mind: Finally your mine and only mine now let's live our life forever together darling.
Y/N and Muzan Lived Happily after that but Y/N keeps having dreams of his past memories. But Muzan reassured him it was nothing. They hunt humans and Demon slayers together and they were inseparable.
The end.