• Aqual Quintessence
• Item Level 1Disenchants into:
  Not disenchantable
• Binds when picked up
• Unique
• Use: Douses a rune of the Firelords.
• 1 Charges

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

Source

Once you complete the quest  [60R] Hands of the Enemy, you can receive an Aqual Quintessence from Duke Hydraxis of the Hydraxian Waterlords in Azshara by talking to him.

 [55] Poisoned Water, 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  [The Fifth Element] was rewarded to players who had obtained an Aqual Quintessence; however, if you had obtained or completed the quest Poisoned Water (which along with  [57R] Stormers and Rumblers is a prerequisite to the quest chain) you may still obtain and complete Hands of the Enemy and get the Feat of Strength.

Notes

• It was used to douse the runes that exist in Molten Core. When all seven runes are doused (one for each boss except Lucifron), Majordomo Executus will spawn.
• The Eternal Quintessence, a permanent, non-consumable version of the Aqual Quintessence, was introduced in Patch 1.11 for players once they have reached revered reputation with the Hydraxian Waterlords. This eliminates the need for players to travel to Azshara every week in order to summon  Majordomo Executus in the Core.

Patch changes

• Patch 3.0.8 (2009-01-20): Aqual Quintessence and Eternal Quintessence are no longer needed to extinguish runes to reach  Majordomo Executus. 
  

External links

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• Wowhead
• Thottbot
• WoWDB