The Shadowlands, also known as the Great Darkness, the Shadow Creature, the Primordial Darkness, the Ultimate Darkness, the Dark Zone, and the Ultimate Evil, is a realm of primordial, quasi-sentient darkness that has existed since before the creation of the matter.
The realm is a power source for beings such as the Shade, Culp, and Obsidian, who utilize the power of shadow in their magical abilities.
When light was introduced unto the world, the Original Darkness retreated and stayed inactive for unknown ages. However, during the Crisis on Infinite Earths, a cult known as the Brujeria awakened the Darkness in hopes to destroy the creator and re-shape the world. John Constantine and the Swamp Thing worked to prevent the awakening alongside a gathering of supernatural heroes as well as parts of the angelic and demonic armies of Heaven and Hell.
The Darkness soon gained consciousness, with several heroes trying and failing to defeat it. Each encounter taught the Darkness a lot and, assessing itself as evil, it pointed its aggression towards Heaven. Finally, Swamp Thing entered the Darkness and tried to explain his theory that good and evil defined each other and that one could not exist without the other.
Coming to accept Swamp Thing's theory, the realm stopped its attack and settled to serve as the shadow off-sider to heaven's light.