Four trials you must pass before you are deemed worthy by the naaru.
Within the Shattered Halls of Hellfire Citadel are three of your people. They are held there by the merciless one, Kargath Bladefist. He seeks to execute them as a sign of his virility and power. You must save these prisoners before such an atrocity can come to pass. As proof of deed, bring me the unused axe of the executioner. It must not have a drop of blood on its blade!
Progress
<A'dal greets you.>
Completion
Well done, <name>. You have passed the trial of mercy.
After defeating Grand Warlock Nethekurse and upon entering the gauntlet afterwards, your group has 80 minutes to reach Warchief Kargath Bladefist. Similar to the quest [60D] Dead Man's Plea - the 45-minute baron run - a timer-debuff appears and Kargath Bladefist will occasionally yell something about executing the prisioners. Although you lose the debuff upon death you still can complete the quest.
The quest description is misleading, as a prisoner or two may be killed and you can still get the quest item - so don't be quick to give up if you miss the deadline, or even a few kills.