Xerrath

Xerrath was a world destroyed by the Burning Legion as a show of power during the enslavement of Xoroth.

Mists of Pandaria
A lost nathrezim codex found in Pandaria contained the instructions for opening a magical gateway to Xerrath. Unfortunately the tome was deemed useless, as the world had already been destroyed and the gateway was too complex for a mortal warlock to open alone.

Trivia
Former systems designer Alexander Brazie, who spearheaded the green fire quest chain during Mists of Pandaria, revealed that he originally intended for warlock players to open a portal to the ruins of Xerrath, where they would do daily quests with the Council of the Black Harvest to drive away the Legion. An incomplete version of the zone, labeled "WarlockIsland," was datamined during the patch 5.2 PTR. The ruins of Xerrath were depicted as a series of floating, cavernous islands suffused with molten fel energy, causing it to strongly resemble Shadowmoon Valley. This island eventually became Dreadscar Rift in World of Warcraft: Legion.