New primary objective: Kill Ennodius Papius
Note! This quest is optional and a chance to get it will show up during completing the quest Sanctuary.
In order to unlock this quest, you have to ask Nazir for new contracts during conversation with him. It is worth noting, that in order to unlock next main Dark Brotherhood quest, Mourning Never Comes, you have to complete at least one of the side contracts (I suggest completing all of them). Leave the Dark Brotherhood's Sanctuary and open the world map. Ennodius Papius stays in the camp in the wilderness, located west from Windhelm (screen above). So it is best to start in Windhelm and walk in direction of Anga's Mill, the former Papius work place.
Since other people shouldn't appear near the Ennodius's tent, you can try to attack him during the day. But it is safer to wait until the night, because he should go to sleep then (screen above).
Crouch and start sneaking towards the tent, where Ennodius sleeps. It is worth using the complete Shrouded Armor, which you've received from Astrid at the end of the quest With Friends Like These (or other, better parts if you have them). Get rid of Ennodius in any way, stabbing him with a dagger for example or shooting an arrow in his head.
New primary objective: Report back to Nazir
You can now return to the Dark Brotherhood's Sanctuary and report Nazir what you've done. If you're going to complete other contracts received from him, you should return to the assassins' hideout only after completing them all. Nazir will reward you with gold.