You encountered Uppermoon 1. At that night he was supposed to meet kaigaku not You Usually people die to him instantly. But You is most likely very different. Now that's the spirit everyone is looking for!

"I like you're fighting style and breathing technique and you're strong will. You're strong. Muzan would like people like you in the demon kind." said Kokushibo. He offered his blood. He would use force if You refused.