Way back in 2005, before I even started playing the game, there were rumors that the first expansion for the instant-classic game would be a trip into the Emerald Dream. The Dream as a concept was briefly touched on in Warcraft III with the introduction of the Night Elves, but the nature of the Dream and the existence of the Nightmare did not come about until World of Warcraft (the same can be said about the Old Gods - who are hinted at but never named in Warcraft III's Frozen Throne expansion.)
So for essentially twelve years (if we count 2004, when WoW launched,) people have been confidently expecting that the next expansion would be the Emerald Dream (ok, for the past two people have been convinced that it would be "The Dark Below," which wound up being a Destiny expansion.)
One of the problems Blizzard has had, though, is that the Emerald Dream is meant to be a version of Azeroth in which there are no humanoids or any of their works or effects. That means a massive continent of Proto-Kalimdor, but also means that there's basically nothing interesting there.
Still, the idea of the Nightmare - a corruption spreading through the Emerald Dream - has been a rather important plot point within the game for a while. The Emerald Nightmare gave us the Dragons of Nightmare world bosses (who disappeared some time before Cataclysm, and not with it, if I recall) and also seemingly had some connection to the Sunken Temple dungeon (which used to be a lot larger.) Eranikus, the final boss of Sunken Temple, was strongly connected to the Nightmare, and there seems to be some implication that Hakkar the Soulflayer, an evil Loa worshipped by the Gurubashi Trolls (and we now know that Loa are the same as Night Elf Ancients and Pandaren Celestials, who are all now grouped under the blanket term "Wild Gods,") is tied to the Nightmare as well.
Warcraft Chronicle also explicitly told us how the Nightmare came to be - druids attempted to create a new World Tree in Grizzly Hills called Vordrassil, which we now know as the Grizzlemaw. It was planted too close to the prison of Yogg-Saron, and the Old God's essence was able to seep into the Dream this way and manifest as the Nightmare.
Making the Emerald Nightmare a raid rather than an expansion-supporting continent has thus allowed them to get away with a large area that doesn't really have "buildings" per se - in fact, what we're seeing is a kind of raid I dreamed up many years ago - a kind of shadowy realm of monstrous beasts.
It's exciting that the Nightmare is finally getting a definitive aesthetic as well - you'll see lots of black and red with perhaps some purple. The Nightmare's connection to the Old Gods is also apparent, such as Ilgynoth, which has very C'thun-like eyes, and also many of the creatures we fight within it resemble the Sha. Yet while it is clearly connected to the Old Gods, the Nightmare looks very distinct - and it will also be a clear visual break from the black-and-green look of the Burning Legion.
We're also seeing the Nightmare Dragons given entirely new models - they're still obviously dragons, but they're very different from the ones we've encountered in the past. (Even if the Dragons of Nightmare, who we fight as a group-boss, are literally dragons we've encountered in the past.)
I'm expecting to find more clarification as we quest through Val'sharah, but I'm beginning to get the impression that while Xavius is deeply tied to the Nightmare and has been empowered by it, it is ultimately a force he intends to use to help the Burning Legion. I'm curious what the Old Gods think of that - the Legion's mission is their destruction, but perhaps the Old Gods revel in any chaos, even that which destroys themselves, and thus are happy to see any spread of their corruption for whatever purpose.
I am somewhat concerned about burning through the Nightmare in a single raid - I hope that it will continue to play a part in WoW to come, but I suspect that we'll never see it quite as front-and-center. Druids will be getting Nightmare-themed weapon appearances for PvP, but I do sort of lament the fact that there won't be Nightmare-themed weapon drops for all classes. (And armor, for that matter.) Perhaps we'll see in time.
Anyway, as I write this it's still Monday, but when this posts Legion should be only one week away. I know I'm very excited to go to the Broken Isles. In fact, for the first time since Cataclysm I'm going into an expansion with no experience in the Beta, and thus it will really be a whole new world for me to explore. I've devised a simple method to randomize the order in which I do the zones (a simple D&D trick called a d4, with the level up zones being numbered west to east.) I'm honestly pretty excited about all of them, but I'm very eager to see how much of Val'sharah is corrupted by the Nightmare.