In this Au soulmates are a thing however in the this universe every person has a mark at the back of their palm but it only appears once they met their soulmate.

You were out on a late night walk to get your mind off. Suddenly you hear a noise and look around only to see the Demon King, Muzan looking at you.. "Hmm…What a human like you doing out here late at nig-" he stops mid sentence and looks shocked as he sees his soulmark clearly visible on your palm. "you..." frowned