The Emerald Nightmare (or the Nightmare for short) is a mobile area of corruption within the Emerald Dream. Despite the defeat of Xavius and the removal of much of the nightmare through the course of Stormrage, the noting of a still-corrupted area indicates that Blizzard has future plans for the Nightmare.
Nature of the Nightmare
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.
The origins of the Emerald Nightmare was not entirely clear, although it is certain that it is somehow connected to the Old God named N'Zoth. While some believe that the Nightmare is born of the Old Gods themselves, others believe that the powerful deity Hakkar the Soulflayer, the ancient Gurubashi Blood God, is the evil power behind the Emerald Nightmare. Hakkar is an old god to the trolls. Dwarven explorer Brann Bronzebeard speculates that Hakkar is an Old God or son of an Old God. The connection between Hakkar, the Green Dragonflight and the Emerald Dream is also hinted to adventurers in the Sunken Temple and, more subtly, in the serpentine theme of the Wailing Caverns.
The reasons why the nightmare has become so strong in recent years was long unknown.
Although Xavius did not create the Nightmare, he came to rule over it it as the Nightmare Lord. Xavius orchestrated the corruption of the Emerald Dream by manipulating Fandral Staghelm into planting a branch of Xavius's own tree into Teldrassil, ensuring that the new world tree would be corrupted from the start. His plans culminated in a daring attempt to bind Azeroth and the Emerald Dream into one plane, seizing control of Azeroth's residents while they slept and giving shape to their nightmares. Xavius himself was a minion of a powerful yet ancient evil (confirmed to be N'zoth). Despite Xavius' defeat at the hands of Malfurion Stormrage and his allies, the Nightmare was not entirely destroyed following its master's defeat. One small part subbornly stayed in the Rift of Aln, and Malfurion could only seal off the vicinity around the rift until a time that another war would be in their favor. The Nightmare's influence in the Barrens following the Cataclysm suggests that the evil controling the Nightmare may have taken advantage of the event to attempt to infect Azeroth once again.
Many druids are called to the Emerald Dream to help fight back its effects. So far, the green dragons have done what they can, though they dare not risk becoming themselves corrupted. The Eye of Ysera has not yet been approached by the Nightmare, but some fear that one day it will.+
In World of WarcraftThis section concerns content exclusive to World of Warcraft.
It is created using the twisted thoughts and fears of sleeping beings’ subconscious minds to corrupt the Emerald Dream. The Nightmare travels slowly through the Dream, leaving corrupted, vicious animals and Unwaking travelers in its path. The Night Elf druid Naralex and his former acolytes in the Wailing Caverns have been corrupted by the Nightmare - transformed into the wicked Druids of the Fang. Eranikus was corrupted by the Nightmare. Even Ysera's most trusted lieutenants (Lethon, Emeriss, Taerar. and Ysondre) have been warped by this dark power within the Emerald Dream. Malfurion fights at Cenarius' side against the Nightmare within the dream, but may have been turned into an Unwaking himself.
In The Burning CrusadeThis section concerns content exclusive to The Burning Crusade.
The Nightmare seems to have found a way into one of the wings of the Stormrage Barrow Dens where strange creatures of nightmare suddenly come in to the waking world. The Cenarion Dreamwarden are constantly trying to keep the threat at bay. Plant life is thriving here, but this is not the case in the Emerald Dream. Clintar Dreamwalker can be found asleep in one of the rooms and he is part of the quest chain for Swift Flight Form. It seems Anzu is behind these attacks.
In Wrath of the Lich KingThis section concerns content exclusive to Wrath of the Lich King.
While on the Icecrown quest  , Remulos sends the player into a phased version of a portion of Moonglade to gather .
During the Valithria Dreamwalker encounter she opens portals to the Nightmare, where you can find dream clouds.
In CataclysmThis section concerns content exclusive to Cataclysm.
The Emerald Nightmare is present in the regrown areas of Southern Barrens. This area is called The Overgrowth and lies just south of the chasm that split the Barrens in the northern west part of the Southern Barrens.
- Malfurion Stormrage says: I fear for the worst, old friend. Within the Dream we fight a new foe, born of an ancient evil. Ysera's noble brood has fallen victim to the old whisperings. It seems as if the Nightmare has broken through the realm to find a new host on Azeroth.
- Eranikus, Tyrant of the Dream yells: My redemption? You are bold, little one. My redemption comes by the will of my god.
- From the novel Stormrage: ...many, regretfully, were like Lethon and Emeriss, and could not survive without the Nightmare; they melted away as the shadow satyrs had. For that and who they once had been, Malfurion mourned them.