Once you obtain the task, the quest window displays the following:
Objective Instruction
Kill 15 Dark Reavers
Kill 50 skeletons
Kill 50 golems
However the 50 skeletons is greyed out as if it is unavailable to complete. Also, when I completed it on Jerant/Hans, I only killed 15 reavers and that was enough. I'll try it again on another character soon once I have another one who will survive the Reaver room :p