New primary objective: Kill Narfi
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. You should look for Narfi in the settlement called Ivarstead (screen above), located in the southern part of Skyrim. Travel there now.
Narfi is a beggar, who inhabits one of the devastated houses in Ivarstead. So do not attack him during the day, unless your character is able to sneak well and attack from a distance. Otherwise you will be immediately attacked by local guards and even if you surrender, you will have to pay a fine or go to jail. It is better to get inside that house during the night, when the beggar goes to sleep.
Crouch and carefully enter the devastated house. 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). Go to the last room, where you'll find sleeping Narfi. Get rid of the beggar 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.