A manaforge operated by the blood elves.
Blood elf camps near a manaforge.
Manaforges in art.
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 Naaru they also took Manaforges.
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: