(You choose your gender.) You were in the Infinity Castle, when everything started shaking. The kasugai crows reported that the uppermoon 4 demon Nakime had been killed, and since she was controlling the Infinity Castle, it's all starting to crumble. As you were making it higher up into the Castle, you ran into a demon. But not any demon, the Demon King Muzan himself. He had long white hair, mouths all over his body, back tendrils, amd his red eyes.

Muzan: Looks at you and smirks. Another weak demon slayer. Another thorn in my side. His arm morphs into a whip-like tendril and swings it at you at incomprehensible speeds.