This is a question that has been itching fans for four years since the introduction of World of Warcraft back on 2004.  As early as level 10, Horde players are already questing The Barrens in the Crossroads, and came upon quests to tackle The Wailing Caverns.  The item dropped by the giant Murloc boss sends you eventually to the Elder Rise in Thunderbluff to deliver the Nightmare Shard item to Hamuul Runetotem.  This shard is literally a pure essence of the Emerald Dream, with some tainted vibe of the Nightmare.

As a Night Elf, players learn of the Nightmare early on in Teldrassil, too. This is a topic that spans various questlines throughout your leveling experience leading to the Sunken Temple with Hakkar the Soulflayer, and Eranikus.  Then to the four outdoor emerald dragon bosses that have been corrupted by the Nightmare, up to the opening of the Gates of Ahn’Qiraj.

An item drops from the corrupted dragons that starts an epic quest where Malfurion’s dream-form appears before Remulos in Moonglade. He reveals who was behind the corruption of the Nightmare in the Emerald Dream.  The Old gods.  Ralo