In World of Warcraft, the Emerald Dream is an alternate reality of sorts, created by the titan keeper Freya to be an image of Azeroth had it not been inhabited by sapient races. All of nature is in perfect balance within this realm, as no intelligent races have inhabited it to change it at all. The entire Dream is the home of the Aspect of the Green Dragonflight, Ysera; she watches over the Emerald Dream and makes sure it is not tampered with.
Emerald Dream Portals
There are five portals in Azeroth that lead to the Emerald Dream. They can be found in The Hinterlands, Duskwood, Feralas, Ashenvale, and Crystalsong Forest on the continent of Northrend. The first four of these portals are guarded by the Dragons of Nightmare, which, in-game, serve as level 60 raid bosses. In can be assumed that in Wrath of the Lich King, there will be another dragon boss that spawns in front of the Northrend portal.
The Emerald Nightmare
The Emerald Nightmare is a corrupted version of the Emerald Dream. Referenced as early as the Wailing Caverns in Vanilla World of Warcraft, the Emerald Nightmare was the first raid of Legion in 2016.