Concept art by d1eselx
Tempest Keep is a crystalline fortress hovering east of the Netherstorm. Constructed by the Naaru, this enigmatic structure also serves as a transdimensional vehicle. The naaru used Tempest Keep as a base of operations to combat the Burning Legion on Outland, but it was taken over by Kael'thas and his blood elves during the War on Outland. Some of the technology on board Tempest Keep was stripped and used to build manaforges, turning it into a reservoir for magical energies harvested from the Twisting Nether.
Surrounding the structure are three satellites which serve different functions. The Mechanar, appearing violet, is used primarily for manufacturing. The Botanica, appearing blue, is used primarily for biological research and experimentation. The Arcatraz, appearing red, is used primarily as a prison for dangerous creatures. A fourth satellite, the Exodar, was hijacked by draenei refugees and used to escape Outland from the Burning Legion.
After his defeat at the hands of Azeroth's armies, Tempest Keep is no longer controlled by Kael'thas or his blood elves. Its current status is unknown, but it is likely reclaimed by the naaru to be used in future campaigns against the Burning Legion.