Necrus, like Atlantis, is another domed underwater city. Unlike Atlantis, Necrus has a more unstable status in the ocean. Necrus only exists on the material plane for brief periods of time. It also has no fixed location, always appearing at different points when it, once again, manifests in the sea. Unbeknownst to the citizens of Necrus, or Atlantis, for that matter, Necrus owes it's existence to an alien satellite that, periodically, enters Earth's orbit, as it moves through the galaxy. Earth's magnetic field triggers a device, built into the satellite, that opens up a hole in dimensional space. From this hole emerges Necrus. The location of Necrus is entirely dependent on the position of the satellite in Earth's orbit, at the time the dimensional rift is formed. As the satellite breaks free of Earth's orbit, the dimensional rift loses cohesion, causing Necrus to fade away.

Necrus is known as "The Black City" It is ruled over by the tyrant king, Mongo. It's society seems to be wholly militaristic. The last time Necrus appeared on Earth, it's citizens slaughtered a surface outpost, that was under construction, in the environs just outside Necrus. Necrus then went to war with Atlantis, for it's continued acceptance of surface dwellers residing in the ocean.