Dream Fog, here observed within the Twilight Grove. Notice that this is how the fog looks like when Particle Density is on full.
Dream Fog is a sparkling green-shining matter which causes people to fall sleep, and is likely related to the Emerald Dream. The Dragons of Nightmare - Emeriss, Lethon, Ysondre and Taerar - use it as a weapon against anyone who dare challenge them. Forcing their mortal foes to fall asleep, the dragons sends their unfortunate victims to the realm of the most terrifying nightmares.
Details
- Dream Fog is summoned by the ability Seeping Fog which is known by all the dragons of nightmare.
- Two clouds will be summoned each time a nightmare dragon summons fog. There are never more than two clouds floating around at the same time. As Seeping Fog will only create one cloud each time it is cast, the dragon will use Seeping Fog twice when summoning.
- Dream Fog is always summoned at the feet of the dragon summoning it.
- The sleep lasts for 4 seconds. If a player is "caught" inside a cloud, the sleep will be refreshed every 2 seconds.
- Although it looks like a spell, a Dream Fog cloud is actually a unit. This enables the clouds to chase players.[1] Apparently, if there are several players fighting the dragon, the cloud will move from one to one, creating a lot of small stuns.[2][3] If there is only one player, a cloud will stop moving once reaching him/her.
- Moving very slowly, a cloud can be outrun by a player moving backwards. Its moving speed is close to walking speed (not running).
- When soloing the dragons of nightmare, this fog is one of the most dangerous abilities they have. Being trapped in the fog can be fatal as it can result in a permanent looping sleep which lasts until the dragon kills you. Because of this, it is a good idea to keep moving in order to outrun the clouds.
- Taerar's Arcane Blast is able to knock sleeping players out of the fog, freeing them. His
[Bellowing Roar] is oppositely able to fear them into it. - While not confirmed, it is possible that the dragons' Tail Sweep ability can also knock players out of the fog (or into it).
- Being a sleep effect, there are few things that breaks the stun. It will not be broken by damage. Abilities that remove sleep effects are PvP trinkets, the forsaken racial
[Will of the Forsaken], and the paladin's 
[Divine Shield]. The hunter's 
[Bestial Wrath] will make both the hunter and its pet immune to sleep effects. - The duration of a cloud is more than a minute. Spell details in WoW-databases (thottbot, wowhead and allakhazam) all describe a 2-minute duration.
- The sleep can also stun player-controlled minions, but the clouds seems to "prefer" players.[4]
- The sleep range of a cloud is around 5-10 yards from the cloud's center.[5][6]
- The sleep can be resisted. It can also miss the player.
