Aqual Quintessence

Aqual Quintessence is a unique quest reward that lets you douse the runes in the Molten Core.

Source
Once you complete the quest, you can receive an Aqual Quintessence from of the Hydraxian Waterlords in Azshara by talking to him.

, one of the two quests which are a prerequisite to the Hands of the Enemy chain was removed after patch 3.0.8, and the Feat of Strength was rewarded to players who had obtained an Aqual Quintessence; however, if you had obtained or completed the quest Poisoned Water (which along with  is a prerequisite to the quest chain) you may still obtain and complete Hands of the Enemy and get the Feat of Strength.