Steal Tazan's key, then use it to open his Satchel and deliver its contents to Therzok in the Cleft of Shadow in Orgrimmar.
I'll leave it to you to get Tazan's satchel open, and return to me with its contents. The locking mechanism is complex, so it would probably be easiest if you obtained the key.
As you didn't find it on Tazan's body, I can only assume that he left the key in the care of another... Ah, yes, that's right. He had an assistant named Gamon. A drunkard, you'll most likely find him at the tavern. Perhaps you could quietly relieve him of the key.