You must slay 4 Sullon centurions. Kill them and speak to the shaman again. They are in the general area of the high shaman, some roam and some are static, around ( -441, -33, -99 ) /waypoint -441, -33, -99. They can also be found in the Defiled Forest in the vicinity of the Sullon Mill. Some spawn in groups of 2 and 3, but singles can be reliably found every 5 minutes at ( -351, -28, -400 ) /waypoint -351, -28, -400.
Return to Tychus to complete the quest and claim your reward.