Provided you got
With Friends Like These..., keep following the Dark Brotherhood missions and complete the questline.
Breaching Security*** Spoiler - click to reveal ***Handed to you by Gabriella. Since you keep getting better at this, the jobs keep getting harder to catch up. This target location is not only time dependent, but it also changes with the days of the week. Luckily you can easily steal Gaius Maro's schedule at the Penitus Oculatus Outpost and anticipate his travels.
Breaching Security*** Spoiler - click to reveal ***Astrid wants you to chase down Cicero. This should lead you to the Dawnstar Sanctuary. Whether you kill or spare him in the end is up to you. The good thing about this quest is that you'll get Shadowmere, a kickass horse.
Recipe for Disaster
To kill an Empire*** Spoiler - click to reveal ***Now this is fun, you have to use disguise and deceit. Plus you'll get to make a recipe with ingredients of your choosing! Too bad this will end with the stench of betrayal. Return to the Sanctuary afterwards.
Death Incarnate
Hail Sithis*** Spoiler - click to reveal ***This is it, you decide to finish this nonetheless. This really feels like a 007 mission: infiltrate the boat, locate the target and execute orders. Return to Amaund Motierre and report back to Nazir. Congrats, you've done it!