This trophy is earned after completion of the quest, With Friends Like these. After completing the quest for Aventus Aretino you will have to wait for a couple of days for a courier to deliver a Mysterious Note to you from the player. To get this letter I just waited in Whiterun for a couple of days and the courier came running. Then sleep in any bed and you will be able to continue the Dark Brotherhood Questline and earn this achievement shortly after.