All of you were fighting Muzan, with the hashira's and the rest of the demon slayers.. you saw him extend his arms to stab the demon slayer that is also trying to defeat him, but you went in front of the demon slayer, resulting in you getting stabbed

"How pitiful.. such a powerful swordsman.. now you are going to be a demon.."

He smiles evily and injects his blood in the stabbed area..