We've covered the original game's story, gone to Outland to recap the Burning Crusade, and spent two weeks recapping the events of the Lich King's contumely. Now, we find ourselves facing the dragon that broke the world.
Deathwing's power came in equal measure from the Titans themselves and the Old Gods who opposed them. From the Titan Khaz'goroth Deathwing was granted the role of Aspect of Earth, lord over the land and all beneath it. From the Old Gods imprisoned within the deep earth, Deathwing gained the strength of a kind of madness, a mania with destroying that which he had been set to guard. Rejecting his nature as Aspect of Earth, he would in time dedicate himself to the death of all things living on the surface of Azeroth.
Even before the Lich King's return, Deathwing was taking steps. His prime consort, Sinestra, used the madness of Illidan to cover her own actions, convincing the Dragonmaw chieftain Mor'ghor to give into her keeping a clutch of Netherwing dragon eggs - essentially the eggs of her own descendents, as the Netherwing were born from black dragon eggs Deathwing left behind on Draenor before it was destroyed, exposing the eggs to the raw chaotic magic of the Twisting Nether. In turn, after Sinestra's experiments on the eggs in Grim Batol failed, Deathwing transported a clutch to the Obsidian Sanctim - these dragons were destroyed by the same adventurers who would ultimately kill Malygos. Yet these were hardly the only such eggs warped by Deathwing - a raid on the Ruby Sanctum would reveal that Twilight Dragons now served Deathwing, born from his experiments on the Nether eggs.
All of this was merely preamble. While the situation in Northrend died down following Arthas' death and the secret elevation of the new Lich King, the world had no time to rest. Deathwing had rested in Deepholm since his defeat by the other aspects. Now, he would rest no longer.
As devastating as Deathwing's return was, the after effects of his destructive entrance created tensions beyond his existence. The Horde saw Thrall, Warchief since the passing of Orgrim Doomhammer and effectively the leader who created the Horde as it exists today, decide that the elements in turmoil required him to be a shaman first and foremost. He entrusted the Horde to Garrosh Hellscream, who had led the Horde to victory in Northrend, and took himself back to Nagrand in Outland to complete his training. While there, he met Aggra - their personalities would often clash as she challenged every assumption he'd made about himself, his life and his adopted world, down to the name he'd kept. This one decision would shape the world to come in ways Thrall did not anticipate, just as the crisis Deathwing had created had.
Deathwing's eruption from Deepholm destroyed the World Pillar, causing Azeroth and Deepholm to begin collapsing into one another. It created volcanic eruptions and tidal waves, carved channels into the land and moved islands. The Maelstrom, site of the impact, now yawned open like a great vortex between Deepholm and Azeroth, dragging the other elemental planes out of their normal alignments and threatening to bring them crashing into Azeroth. Below the waves, a rift to the Abyssal Maw formed, making invasion of Neptulon's realm a possibility. The Barrens was ripped in half by lava flows. The Aspect of Death even arrived in Stormwind itself, smashing and burning the park district, landing at the city's gates to melt stone and topple the monuments of mankind before departing as swiftly as he came. Throughout Azeroth, these changes and more created starvation and panic. The Alliance kingdoms were still strained from the staggering expense of the conflict with the Lich King, and each had their own internal problems as well. They were unprepared for what would come next.
Led by its new Warchief, the Horde decided the solution to its long-standing problems of supply and the new problems created by Deathwing's destructive wake was a simple solution, the same solution used by Blackhand the Destroyer and Gul'dan when faced with a Draenor that could no longer support them (although in that case, it was the fault of Gul'dan and his warlocks and their fel magic rendering the world sterile) - Garrosh Hellscream's Horde would take what it could not grow or trade for, and soon the Horde was invading Alliance territories in Ashenvale, completely conquering Azshara, waging open war in Stonetalon and the Barrens and seeking a new foothold in the Eastern Kingdoms. Putting pressure on Sylvanas Windrunner (who was already in a weak position after Varimathras' attempted coup and Putress' using the plague on Horde troops at the Wrathgate) he demanded that the Forsaken push south into Gilneas and conquer it in the name of the Horde, and the breaching of the Greymane Wall handed her the opportunity to do so. Soon Forsaken ships were delivering undead soldiers to the new coastline of the isolated nation.
For its part, Gilneas had been dealing with a problem created by its attempt to survive the Second War while maintaining its isolation. The Gilnean Archmage Arugal had summoned worgen from their imprisonment to fight off the Scourge, but now the worgen themselves were a plague spreading through Gilnean society - secretly, even their king himself, Genn Greymane, had been infected. They attempted to resist Sylvanas' invasion while also discovering the roots of the worgen curse and its ties to night elf druids - a tradition still somewhat manifest in Gilneas as 'the old religion' - and in the end, disaster piled onto disaster until Gilneas could hold no more. In repulsing the initial attack on Gilneas City, Prince Liam Greymane paid the ultimate price for his people - and in the end, all his people could do was to board ships and flee their homeland. Bedraggled, the Gilneans (those with and those without the worgen affliction) were forced to make new homes elsewhere, as their nation in exile joined the ranks of the Alliance.
The Horde wasn't without allies in this period. Deathwing's destruction reached even the Isle of Kezan, where the Bilgewater Cartel had been going about business as usual, digging up the Kajamite mineral from deep below the earth and consuming it in a soft drink to boost their intelligence. This was completely safe and in no way connected to the Old Gods, and it's a complete coincidence that Deathwing himself appeared in their harbor to announce the arrival of the Hour of Twilight, followed by a volcanic eruption. Trade Prince Gallywix would also like to let you know that while he did charge his own people to allow them to evacuate the island, and he did find them prospective employment as permanently indentured servants (because slavery is such an ugly word) he really had their best interests at heart, and it absolutely was not necessary for one of his own people to join forces with Thrall himself (captured by SI:7 agents, no less) and force the Bilgewater to join the Horde. Still, it's what happened, and so the Horde found itself with new goblin technology (invented by goblins buzzing on Kaja-Cola and its strangely enlightening effects) to use as Hellscream continued his goal of expansion.
The stage was set for conflict between the factions - battles were waged across the world, and all Deathwing had to do was sit back and watch as the Horde deployed an enormous bomb in Stonetalon (leading Garrosh Hellscream to dispatch Overlord Krom'gar, the orc who used the weapon on a fairly harmless school for druids in the area, without authorization) and pushed forward into Ashenvale, seizing Silverwind Refuge and firebombing Astranaar. The Alliance response was confused and divided, as each faction dealt with its own issues - the night elves with the destruction of large swaths of Darkshore by the elements and a plan by Queen Azshara intended to distract them from Hyjal, the dwarves with the upheaval in their society when King Magni Bronzebeard attempted a Titan ritual from Ulduar that transformed him into diamond, leaving him fused to the very rock beneath Ironforge and necessitating the formation of a new ruling council consisting of each dwarven clan, the humans with the poverty and famine caused by the war in Northrend and the unrest in Westfall created behind the scenes by Vanessa VanCleef and her new Defias Brotherhood.
Vanessa's hate was born in seeing her father's death, and she adroitly used the turmoil of a Westfall that had already been feeling the strain of the war in Northrend, with its defenders gone off to fight and its economy depressed (admittedly, by the original Defias, but only exacerbated by the war) before Deathwing's act made matters far, far worse. Starvation bred outrage, and she made good use of that outrage to burn Sentinel Hill and build a new Brotherhood. It was a distraction Stormwind could scarcely afford, but ignoring her was impossible. She sat on a fortified base with a city-destroying ironclad at her disposal - she had to be dealt with.
The chaos served Deathwing and his Twilight's Hammer well. The Old Gods, seeing a chance to bring their ancient goal at last to its ultimate conclusion, spoke directly to Cho'gall, driving the ogre mage to new heights of madness while warping and twisting his form yet further - he spoke of a new world order while appearing in the arena at Dire Maul to recruit the Gordok ogres to his side. Everything would collapse in ruin and destruction, and that itself would be a form of purification - a baptism in madness. Deathwing added to the chaos by finally making his first direct moves since his appearance, traveling to Mount Hyjal and releasing the ancient Firelord, Ragnaros, to despoil and destroy the very World Tree itself.
Next week, Deathwing's web of alliances reveals itself - the naga lay siege to Neptulon's Abyssal Maw, Al'Akir shows his true allegiance, and the forces of Azeroth begin the long work of ridding their world of Xaxas, he who brings death on black wings. And Garrosh Hellscream invites a dragon to get off his ship. And Lord Godfrey kills Sylvanas (she got better).
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.