The Emerald Nightmare (or the Nightmare for short) is a mobile area of corruption within the Emerald Dream.
The Emerald Dream is not a place of total perfection. It can be shaped and twisted by dreams, usually without a sleeper’s knowledge. Sadly, nightmares are themselves a crafting of the subconscious and have given rise to their own place within the Dream. The Nightmare, as the denizens call it, is not a specific location, but a constantly roaming effect that travels about unpredictably. The skies above the Nightmare are constantly clouded, creating a gloomy, depressing zone that weakens and deadens everything it touches. The beautiful green vegetation is twisted into brown decay, while ordinarily peaceful creatures are corrupted into shadowy, mutated versions of themselves known as Corrupted Ones. The Corrupted Ones are vicious and cruel, hunting anything they can, even after they leave the Nightmare’s affected zone.
Worse still, some dream-travelers who enter the Nightmare — or are engulfed by its movements — become trapped in the Emerald Dream. They cannot leave, so their bodies back on Azeroth remain in a permanent sleep, slowly starving to death unless tended by helpful aides. While trapped, they are known as the Unwaking; they are also corrupted, and will do anything to get back home. The Nightmare is believed to be slowly growing in size and power.