The Underworld was a realm that was the home of (and ruled by) the Morlocks.
The Underworld was a dark, dense, and never-ending cave-like structure with countless stalactites and stalagmites along with ledges. Also, numerous depressions and downward spiraling tunnels frequently occurred. At the very bottom of the largest tunnel sat the lair of Octomus where Necrolai, Koragg, Morticon, Imperious, the Styxoids, and the Hidiacs resided.
At the center of the throne room was a white pool that turned red and signaled the Master's presence. The lair also hosted numerous passageways and corridors including lower levels. The lowest of all the levels was the Inner Sanctum where the Ten Terrors resided.
Twenty years ago, Octomus from the depths of the Earth, attempted to escape in order to conquer dimensions. In a climactic war between the Morlocks and the Mystics, Leanbow sealed them away away once more.
Twenty years later, Koragg used Clare's Gatekeeper powers to raise the Underworld and unleashed Morticon. After his defeat, the Underworld sank away.