The fastest way to get this attire to kill him and loot the item afterwards. He’s located in the Dark Brotherhood Sanctuary. And the fastest way to enter the Sanctuary is to do the quest “Destroy the Dark Brotherhood!” Take note that doing this quest will prevent you from joining the Dark Brotherhood, and prevent you from doing its associated quest line.
Getting to the Sanctuary
Start with the quest “Innocence Lost”, and then do “With Friends Like These…”. During this quest, instead of killing the captives, kill Astrid to start the quest “Destroy the Dark Brotherhood!” You can report Astrid’s death to any guard, and they’ll tell you to report it to Commander Maro in Dragon Bridge. You can find him at the Penitus Oculatus Outpost. Go there and talk to the commander, and he’ll give you the password to the door, and assign you to infiltrate the Sanctuary with the goal of killing the members of the Dark Brotherhood. Now you can travel to the Sanctuary of the Dark Brotherhood, located west of Falkreath. When you get there you can use the password given by Commander Maro to get inside (just go up to the door and it should ask for the password). Once you’re inside, just kill all the members (including Nazir) and then loot the unique Redguard Clothes and a Redguard Hood from him. You’ll then want to return to Commander Maro to complete the quest.