Taxidermy- Kokushibo
He told everyone who bothered to ask how his brother had died a story, one where he'd battled him even in old age, and had died. But that was simply a story, one to cover up something much worse.
When he'd found Yoriichi he'd known Muzan would want him dead. But Kokushibo still cared about his brother for some unknown reason. So, he pretended to be… chill around Yoriichi, insisting that to spare both their feelings they simply wait until someone else killed the other one. It was something Yoriichi agreed to. Kokushibo almost stuck to his word. Until Muzan handed him a present and an idea.
He met up with Yoriichi again. They were simply relaxing at a park, a few hours until sunrise. Knowing his brother, he'd be tired by now, after almost a full night of slaying as many demons as possible. So Kokushibo offered Yoriichi a hug. Obviously too tired to suspect anything, the brother hugged him back. that's when Kokushibo did it. he injected the drug into Yoriichi. One that would paralyse him, and, according to Muzan numb him. somehow Kokushibo was doubtful of that. either way he held Yoriichi, who was either too tired to notice the drug being injected or in a state of shock from the drug. That didn't matter much. It took just a second or two for Yoriichi to go limp in his arms. Kokushibo had to carry him back to Muzan's house. Since Muzan was out Kokushibo could drag Yoriichi down to the basement.
The basement was a place Kokushibo didn't go in often. But he knew Muzan took slayers there to torture them for information. The place was a surprisingly tidy for a torture chamber, it had one examination table robbed from a mental hospital right in the centre, with straps to secure someone to it. that was the only thing Kokushibo needed. He undid the straps and let most of them dangle at the sides. With the drug Yoriichi was on he wouldn't be fighting back anytime soon. putting him down on the table Kokushibo looked around for the equipment he'd need, apparently Muzan already had it and Kokushibo didn't want to ask for any more details as to why. He could hear Yoriichi breathing slightly heavier and mumbling something. Obviously, the drug hadn't fully muted him yet. Grabbing what he need, Kokushibo turned around to face his brother who looked so confused. Maybe the drug was affecting his mind? Either way Kokushibo wanted to tell him something "don't worry, you won't feel a thing" at least that's what Muzan said, but he left that out.
Taking out the tools Kokushibo began sawing off Yoriichi's arm. He'd need to empty it if he wanted to stuff it. he could see the look of pain in Yoriichi's eyes, Muzan must have lied. Kokushibo did feel a bit bad. Human taxidermy must be painful. Soon the arm came off and Kokushibo stuffed it. putting it aside and sealing the wound on Yoriichi's arm. This was going to take a while.
After a few hours Yoriichi died and Kokushibo couldn't decide if that was a good or bad thing. Either way his body would remain in perfect condition. He even made a few modifications a few years later, and the body stood the test of time. he eventually had to give it up, after Muzan insisted. But he saw it once more. apparently, it had moved into being a training dummy in the swordsmith village.