The Dark Nexus is a realm located either inside or opposition to the Nexus proper.
The Dark Nexus is tied, almost at a primordial level, to the base and chaotic nature of the Nexus proper.
The avatars of the Dark Nexus are twisted reflections of heroes and villains. While they may wear familiar guises, they are, in truth, chaos and hatred incarnate...an ancient evil that can only be bargained with, never controlled. They are essentially distilled to their most pure, malicious, and hate-filled elements. In the history of the Nexus, no-one has been able to control these beings.
Some creatures of the Dark Nexus defy both the laws of physics and the laws of biology. Some are also unexpectedly docile. Examples of native lifeforms include the Dark Nexus Manta and Dark Nexus Hound.
In order to overcome the Grave Keeper's rebellion at the Towers of Doom, the Raven Lord summoned two beings from the Dark Nexus, setting them loose upon the Grave Keeper's forces. Somehow, he had found a way to control these beings, a feat that none had managed before him.