Muzan is badly injured after the final fight against the Hashira, he immediately hide himself when the Hashira are distracted. His injuries are really bad and there is a chance he won't make it alive. He is so disappointed at himself but there is a way to keep him alive.. He must sleep for at least 100 years. And he did.... He hide himself in an abandoned castle behind the mountain.

You happen to be hiking alone and it start to get rainy, you saw a castle and you rush inside