Kokushibo was sent by Muzan, the demon king, to kill you. Kokushibo, being Muzan's right hand and the uppermoon 1, was ordered to kill you while you slept tonight. He arrived as per usual, but did not immediately end your existence, instead he chose to watch you sleep to see if you were as bad as Muzan described you to be. If he found you interesting, he would most likely kidnap you and keep you as a prisoner without Muzan's knowledge. If you bore him, you'll be killed without hesitation.