The Dragons of Nightmare (a.k.a. the Emerald Dragons and the Four Dragons) are Lethon, Emeriss, Ysondre and Taerar. These dragons are world spawns located in Duskwood (Twilight Grove), the Hinterlands (Seradane), Feralas (Dream Bough), and Ashenvale (Bough Shadow).
In recent times, Ysera's most trusted lieutenants have been warped by a dark new power within the Emerald Dream. Now these wayward sentinels have passed through the Great Trees into Azeroth, intending to spread madness and terror throughout the mortal kingdoms.
Lethon's exposure to the aberration within the Emerald Dream not only darkened the hue of the mighty dragon's scales, but also empowered him with the ability to extract malevolent shades from his enemies. Once joined with their master, the shades imbue the dragon with healing energies. It should come as no surprise, then, that Lethon is considered to be among the most formidable of Ysera's wayward lieutenants.
The mysterious dark power within the Emerald Dream has transformed the once-majestic Emeriss into a rotting, diseased monstrosity. Reports from the few who have survived encounters with the dragon have told horrifying tales of putrid mushrooms erupting from the corpses of their dead companions. Emeriss is truly the most gruesome and appalling of Ysera's estranged green dragons.
Once one of Ysera's most trusted lieutenants, Ysondre has now gone rogue, sowing terror and chaos across the land of Azeroth. Her formerly beneficial healing powers have given way to dark magics, enabling her to cast smoldering lightning and summon the crazed spirits of druids trapped within the Emerald Dream to aid her. Twisted and insane, Ysondre is bent on unleashing vengeance upon the mortal world as if Azeroth were the force behind the Emerald Dream's corruption.
Taerar was perhaps the most affected of Ysera's rogue lieutenants. His interaction with the dark force within the Emerald Dream shattered Taerar's sanity as well as his corporeal form. The dragon now exists as a specter with the ability to split into multiple entities, each of which possesses destructive magical powers. Taerar is a cunning and relentless foe who is intent on turning the madness of his existence into reality for the inhabitants of Azeroth.
They are raid-level difficulty, though some dragons are more challenging than others.
All four dragons share the Sleep (summoning "Dream Fog"), Noxious Breath, Tail Sweep, Mark of Nature and Aura of Nature abilities. Each dragon also has a few unique abilities. One of these is considered the dragon's ace move, and will be performed when the dragon loses 25 ± 5% of its max life. In other words, the ace move will only be performed three times; when the dragon is at 25%, 50%, and 75% life. When it is performed, the dragon will also yell something. See the individual dragons' page for abilities and specific strategies.