The Great Dark Beyond, also known as Great Dark, Great Beyond, Dark Beyond, or simply Beyond, is the empty void between worlds. It is described as the first of everything when there was nothing, and is part of the primary Material Plane, as opposed to the Twisting Nether. It is essentially the name given to outer space in the Warcraft universe.
To date, five planets have been specified in the Warcraft universe: Argus, Azeroth, K'aresh, Xoroth, and Outland (formerly Draenor). However, countless other planets exist in the Great Dark Beyond.
When debates on cosmology began on Azeroth, scholars quickly tumbled to an obvious question: where does it end? If Azeroth is surrounded by the Emerald Dream, and separated from other material worlds by the Twisting Nether, what lies beyond the boundaries of the Nether? The result eventually agreed upon, as much a philosophical placeholder as an answer, is the Great Dark Beyond. Neither black nor white, hot nor cold, living nor dead, with a beginning hard to define on the chaotic fringe of the Nether and an end impossible to define: the Beyond is all that is unknowable about the cosmos. Were part of it to be journeyed or defined, it would become its own plane — leaving the Beyond on its horizon, eternal and all encompassing.
Great Dark Beyond vs. Twisting Nether
The Twisting Nether is the Astral Plane between worlds. It is a chaotic and magical environment that overlaps with the Great Dark Beyond, yet is normally imperceptible to mortals. The roles of both have not been clearly confirmed yet. For example, in the novel The Last Guardian, Medivh explained to Khadgar that demons were denizens and rulers of the Great Dark, and referred to the Dark Portal as a gateway that bridged two nodes residing in the Twisting Nether. Later Warcraft material constantly referred to demons dwelling in the Twisting Nether. However, if the information Medivh gave the young Khadgar is to be considered accurate, then the Twisting Nether could be an alternate dimension or a "hyperspace" of sorts where the conventional laws of physics, time, and space do not apply, and thus allows for instantaneous intergalactic travel.
Aegwynn, in her narrative at the start of the Second War, referred to the Great Dark in much the way later sources established the Twisting Nether, so we can assume it was an error or a retcon. The usage of Nether and Great Dark was more ambiguous in Warcraft II than in later games. Though Aegwynn claimed to be responsible for defending Azeroth from forces in the Great Dark, later sources clearly established that her duties dealt with the Nether.
It is possible that Aegwynn chose to use Great Dark instead of specifying the Twisting Nether because her duties extended to defending Azeroth against harmful entities that are not solely restricted to the Twisting Nether. Indeed, it is chillingly possible that there are forces in the Great Dark that rival the Burning Legion and are yet unknown to the powers that vie for Azeroth.
The Warcraft RPG implies that the Twisting Nether is a smaller part of the Great Dark, but that there is no specific definable boundary between the two. This was likely made to explain why the terms are sometimes interchangeable. The Warcraft Encyclopedia now confirms that the Great Dark includes the Twisting Nether, rather than vice-versa.