Slay The Spectre of the Feerrott (level 39-40). He is spawned by proceeding to the final area of the instance and finishing an event at the stone altar. Clear all of the skeletons worshiping at the altar to initiate the event. Note that the normal creatures do respawn, so you must plan around them. If the worshipers are not attackable then proceed to kill every skeletons in the proximity.
Return to Mashara or Tarin to receive your reward.