There are worlds we have not yet visited. Realms we have not seen, do not know. Some are demon-haunted nightmares, still others lost to our understanding, destroyed. The Burning Legion has been thwarted twice on Azeroth, but make no mistake - they have yet to bring their full power to bear, seeking instead to seize the world's unique magical energies for their own use. The dead world Xerrath demonstrates that the Legion has in the past destroyed entire worlds merely as a show of force to cow others into submission and slavery. What's worse is, there are races that were wiped out by them, but others who proved their worth were instead made into servants and slaves to the Legion - and Azeroth has balked their advance twice, putting us squarely in the strong enough to make good slaves category.
Yet, too, there are worlds beyond even the Legion's control, worlds dominated by the vast hosts that the Old Gods and their servants such as the Prophet Skerram, worlds like K'aresh (home of the Ethereals), worlds shaped by the Titans, and worlds that yet sleep in their infancy, unaware of the struggle between ancient inimical forces. Azeroth was one such world not too long ago. The destruction of K'aresh by Dimensius the All-Devouring proves that there are forces, hungry and ravening, that serve no masters and have destruction on their mind completely unrelated to the Legion or the Old Gods.
We know barely a fragment of the vast cosmos. Let us explore what little we do know.
This world is the sole remaining one of a twin planet system, the destroyed twin being Xerrath. Based on what we know from this quest, it's possible that Xerrath was the world that the Nathrezim first contacted and the reason they destroyed that planet was possibly because they'd already enslaved and/or converted everything worth preserving already. It would fit the usual Nathrezim pattern of corrupting societies into waging genocidal wars on each other instead of trying to conquer a world with brute force. It's also fascinating that a book written in Nathrezim was found in Pandaria, of all places, implying that someone was already summoning the Dreadlords to Azeroth well before the Legion invasion. Even more interesting is the revelation from that very questline that the Doomguards, far from being just another demon, were in fact servants of the Titans hunting those who used blood sacrifices in their magical workings.
The reason this is interesting is it implies a connection between the book being found in Pandaria (known home of Titan facilities and the mogu, former Titan constructs) and the world of Xerrath. Why was a book that allowed for travel to the dead world in Pandaria, written in Nathrezim? Were the Nathrezim traveling to Azeroth from Xerrath... or were they traveling to Xerrath from Azeroth?
At any rate, Xoroth itself was pacified by the Legion by the simple expedient of destroying Xerrath as a show of force. Xoroth itself endures to this day, and may in fact be one of the worlds that the Legion's forces retreated to when the Sundering destroyed the portal Sargeras used to attempt to enter the world, for the planet Xoroth is apparently home to satyrs, who as we know were created by Sargeras from night elves like Xavius. The fearsome dreadsteeds summoned as mounts by warlocks are also originally from Xoroth, where they are kept by the dreadlord Hel'nurath. (The fact that Hel'nurath, a prominent Nathrezim, seems to be in charge of Xoroth makes the connection to the ancient text found in Pandaria all the more interesting.) Xoroth itself seems to be a base for the Legion, but we know little else - why did they want to force the surrender of its inhabitants rather than destroying them outright? Why didn't they corrupt them as they generally tend to? Was Xerrath truly just destroyed as a show of force or was there another reason for its destruction?
K'aresh is a world stripped of the ability to sustain life as we understand it by a cosmic force as inimical to life as any Legion demon, the Void. Once a desert world with several intelligent species on its surface, the fate of the planet was sealed by the appearance of Dimensius, the All Devouring. This void lord arrived on K'aresh through unknown means (possibly summoned there) and once he arrived, he began opening countless portals to the Twisting Nether in an attempt to summon the entire voidwalker race to the world to help consume it. The ancestors of the modern ethereals used their brilliant intellects to craft a defense against the energies leaking into their world, but it only worked against the shadowy void energies, not the raw, uncontrolled arcane might of the portals. This led to their bodies being effectively shredded into pure arcane power, leaving them beings of mind who must wrap themselves in treated bandages to help maintain their physical presence.
Dimensius eventually rendered K'aresh lifeless. It may or may not still exist, orbiting the twin suns that made it such a warm world - if it does exist at all it is as a barren rock scoured by arcane storms, battened upon by countless legions of voidwalkers and void elementals. While Dimensius perished in Netherstorm, there are other void lords leading the Void, and although it hasn't been fully explicated there is clearly a relationship between the naaru and the voidwalkers, as a naaru who falls too far out of balance between his light and dark life cycles can become a void god such as Entropius. It's feasible that beings like Dimensius and other void lords are in fact 'born' out of this naaru life cycle.
This world has never actually been visited by any of the Azerothian races, nor has anyone from Azeroth ever met anyone or anything that even reported to be from there. It was a world utterly destroyed by the Burning Legion in a vision of the distant past Velen the Prophet saw of its fate. Fanlin'Deskor isn't even the planet's real name, it is a draenei phrase for the world Velen used to describe it - "Amber skies over wondrous rock" for its varied geology.
The Legion utterly annihilated this planet. Nothing survived.
While we have no idea of this planet's current fate, over 25,000 years ago Argus was a thriving planet. The homeworld of the original eredar before the coming of Sargeras, it never experienced Nathrezim corruption or saw invasions of void entities such as Dimensius - indeed, at some distant point far before the fall of the eredar it apparently had contact with the naaru, based on the presence of the Ata'mal Crystal dating back to well before Velen's own life.
Argus was apparently a beautiful world with many mountainous regions. The draenei of the Bloodmyst Isle settlement still view Mac'Aree, the long-lost capital of Argus, with wistful fondness for its beauty. The world itself may still endure, as the bargain made with Sargeras by Kil'jaeden and Archimonde may have preserved it from the wrath of the Legion. Indeed, considering that the eredar became the backbone and overall commanders of the Legion entire, it's likely that they preserved Argus as the first world to ever willingly join Sargeras' crusade for universal annihilation. Of course, it's just as likely that the eredar, now wholly corrupt, destroyed their own planet.
At present, along with Azeroth and Outland, these are all the worlds we yet know of in the Great Dark Beyond. What lies beyond is unknown... yet. We don't know of the worlds the Prophet Skerram spoke of, we have no idea if the Nathrezim congregate on Xoroth or in some other world (for that matter, we don't know where the prison Sargeras once placed them within and later freed them from is) and where the Titans are now, we have no idea. The Great Dark Beyond is indeed dark to us.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.