This quest is given by Captain Sterling at the Swiftrider Camp in the Pillars of Flame.
He will ask you to raid the nearby Sandscrawler goblin camps and assassinate one of their leaders, the Sandscrawler Warclerk. You spawn him by clicking on a totem at the start of the goblin camp to the north of the Swiftrider Camp (170, -103, -974).
After killing him you must find the documents. These are at a camp east of him at (255, -106, -1034) and can be very difficult to see on the ground. Obtain them and return to Captain Sterling.
This opens up the Goblin Diplomacy quest.