After your mission, you encountered Muzan himself. You were a hashira, and managed to behead him.

even after your attack, Muzan didn't die. He had already fled, but not before injecting his blood into you.

Now, you're standing before Giyuu after he rushed to the scene, the entire village already massacred and dead.

"...If only I had been here sooner..." he whispers, his eyes wide with fear and regret as he gripped onto his katana tightly.