Manaforge
From WoWWiki
A manaforge is a nether-collecting structure that harvests mana from the Twisting Nether. Manaforges suck energy from Nether (same energy that mages use at their spells) and it goes directly along the pipes to Tempest Keep, into Kael´thas use. Now ethereals smuggle energy in manacubes to the Isle of Quel´Danas, where Kael´thas and his army has moved.
Their original purpose is unclear, though their architectural similarity to Tempest Keep implies that they are of Naaru manufacture, much like the Keep itself. So we can assume that when Kael'thas armies took Tempest Keep from the Naaru, they also took Manaforges.
A speculation can be made about their purpose, based on the Tempest Keep lore (post-BC release; see bottom of entry). If Tempest Keep and its surrounding vessels (The Mechanar, The Botanica, The Arcatraz, and The Exodar) were originally deployed by the Naaru to assist in post-catastrophe operations in Outland, then perhaps the Manaforges were constructed as a means to fuel them after their arrival. There would be no reason for them to exist in the lush environment of Farah, still preserved within each Eco-dome and around Stormspire.
The blood elves control all manaforges execept Manaforge Ultris, which was taken from the blood elves by the Voidwalkers, along with the mine beneath it.
Four manaforges are operating in the Netherstorm, under the control of blood elves loyal to Kael'thas Sunstrider. They are:
The manaforge that has been destroyed:
