I've recently been playing through Skyrim trying to get all of the achievements finished and since I had never had the chance to join the dark brotherhood I decided that now I've finished all the DLC I would try to join up to get the last few achievements I need for the game.

I've been trying to get this mission I need to do before I join the dark brotherhood but I can't get it to start. I'm not sure if its because I left it so long after finishing the game and the DLC or maybe has someone else had this problem and figured out how to fix it???