New primary objective: Kill Ma'randru-jo
Note! This quest is optional and a chance to get it will show up during completing the quest The Silence Has Been Broken.
In order to unlock this quest, you have to ask Nazir for new contracts during conversation with him. It is worth noting, that this contract and others will be unlocked only if you have completed all previous side contracts. Leave the Dark Brotherhood's Sanctuary and open the world map. Khajiit, whom you must assassinate, travels with a caravan, which means that there is no one specific place of his stay (a caravan stops near Markarth and Solitude among others). So you have to determine current location of the caravan (fortunately it doesn't move all the time), travel to the nearest discovered location and then reach the Khajiits camp (screen above).
Another problem with this contract is fact, that Ma'randru-jo almost never goes to sleep, so he can't be eliminated in similar way as previously murdered persons. So I suggest to wait for the night (screen above), because it will reduce the risk that your actions will be noticed by guards patrolling this area.
Depending on your preferences, you can act in two main ways. You can attack all Khajiits and deal with them in direct fight or focus only on eliminating Ma'randru-jo. In the latter case it is good to use a bow (screen above), preferably with poisoned arrows, so you would need only one lucky hit. An advantage of this solution is also fact, that after eliminating Ma'randru-jo you can quickly flee, avoiding the fight with the rest of Khajiits.
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.