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 were originally not entirely clear, although it was certain that it was somehow connected to the Old Gods. Some believed that the Nightmare was born of the Old Gods themselves, while others believed that the powerful deity Hakkar the Soulflayer, the ancient Gurubashi Blood God and possibly the son of an Old God, is the evil power behind the Emerald Nightmare. 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.
During the War against the Nightmare, it was revealed that the the Emerald Nightmare was under the rule of a figure known as the Nightmare Lord, who in fact was really Malfurion Stormrage's old nemesis Xavius. However, it was also revealed that Xavius himself was a minion of a powerful yet ancient evil, residing in the Rift of Aln, and possibly also beneath Azeroth's oceans. This evil is the Old God N'Zoth.
World of Warcraft
This section concerns content exclusive to World of Warcraft.
The Emerald Nightmare was 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. Many druids were called to the Emerald Dream to help fight back its effects. The green dragons did what they could, though they dared not risk becoming themselves corrupted. The Eye of Ysera had not yet been approached by the Nightmare, but some feared that one day it will.
The Night Elf druid Naralex's former acolytes in the Wailing Caverns were corrupted by the Nightmare - transformed into the wicked Druids of the Fang, while Naralex himself slept, trapped within the Nightmare. The Green Dragon Eranikus was corrupted by the Nightmare. Even Ysera's most trusted lieutenants (Lethon, Emeriss, Taerar and Ysondre) were warped by this dark power within the Emerald Dream. Malfurion fought at Cenarius' side against the Nightmare within the dream, but was eventually turned into an Unwaking himself.
World of Warcraft: The Burning Crusade
This section concerns content exclusive to The Burning Crusade.
The Nightmare found a way into one of the wings of the Stormrage Barrow Dens where strange creatures of nightmare suddenly came in to the waking world. The Cenarion Dreamwarden were constantly trying to keep the threat at bay. Plant life was thriving here, but this was not the case in the Emerald Dream. Clintar Dreamwalker could be found asleep in one of the rooms and he was part of the quest chain for . It seems Anzu was behind these attacks.
World of Warcraft: Wrath of the Lich King
This 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 the player can find dream clouds.
World of Warcraft: Stormrage
During the events of the War against the Nightmare, the Emerald Nightmare was almost able to take hold of the whole Emerald Dream and Azeroth at the same time. However, the Nightmare Lord was eventually defeated with the help of a coalition of races, stopping the crisis.
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 stubbornly 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.
World of Warcraft: Cataclysm
This section concerns content exclusive to Cataclysm.
The Emerald Nightmare is present in the Nightmare Scar, located at the center of the regrown area of Southern Barrens known as the Overgrowth. The Nightmare's influence in the Barrens following the Cataclysm suggests that the evil controlling the Nightmare may have taken advantage of the Shattering to attempt to infect Azeroth once again.
The Nightmare is present in the dream around the Hyjal Barrow Dens, trying to prevent Fandral Staghelm's transfer out of the prison.
Dawn of the Aspects
During a meeting of the Wyrmrest Accord, Merithra notes that the Nightmare is stirring within the Rift of Aln and seems to be seeking a new Nightmare Lord to replace Xavius. However, Ysera says it is now the duty of the Druids to watch over the Emerald Dream as she has become mortal following the Hour of Twilight.
- 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.
- ^ a b Arthaus. Shadows & Light, 147. ISBN 9781588469731.
- ^ Malfurion#Conversation_with_Remulos
- ^ Arthaus. Lands of Conflict, 59. ISBN 9781588469601.
- ^ Stormrage, page 389
- ^ As revealed by Chris Metzen during the BlizzCon 2010 Quests and Lore Panel
- ^ World of Warcraft: Stormrage, p.389
- ^ Dawn of the Aspects Part 1, chapter 1