This quest is a RE-OCCURING quest that involves talking to the NIGHT MOTHER and receiving a contract from a random person throughout Skyrim. The person has performed the Black Sacrament, and will ask you to kill a target (also a random target). Once you kill said target the quest will both END and BEGIN again. You know how to kill by now, just be sure you don’t get caught (keep saves before you assassinate) and make the Dark Brotherhood proud.