Deathwing's Last Son:
When Deathwing emerged from Deepholme, he unleashed devastation across the planet. He allied with Twilight's Hammer and sought to destroy the world - either to free or escape the Old Gods (the latter being my fan theory.)
Deathwing was becoming something quite unlike a dragon, and by the time we destroyed him, he had broken apart into some kind of elemental aberration. His brood, the Black Dragonflight, played a somewhat smaller role in the expansion than his new invention, the Twilight Dragonflight. But the old Black Dragonflight was still a presence, particularly in the central Eastern Kingdoms.
It was in the Badlands that we came across a goblin woman named Rhea, who eventually revealed to us that she was Rheastrasza, a Red Dragon who had journeyed there to try to see if there was a way to redeem the Black Dragonflight and free them of the Old God corruption that had spread through them from Deathwing downward.
In the course of your quests there, you manage to use Titan technology to synthesize a Black Dragon egg, using the remains of dead whelps and some corrupted eggs. Rhea sacrifices herself to make Deathwing think the egg has been destroyed, but in fact she has an ally hide it away with her flight.
There's a gap in the story here for most characters, but Rogues have a chance to get the next chapter of the story.
Intercepting a communiqué for Twilight's Hammer, you find out that the egg was taken to Twilight Highlands, and that forces are coming to steal the egg. You arrive too late, finding that the egg has already been taken. You track the egg to Ravenholdt - a familiar location to Rogues from before Cataclysm.
After sneaking into the manor house, you discover that not only has the egg hatched, but that it was the dragon whelp inside who orchestrated his own extraction. Wrathion has more or less taken over Ravenholdt for himself, and while he shares a cause with the Red Dragons, he has no interest in being their slave. Wrathion asks you to work for him instead.
His goal is the complete extermination of his own family, himself obviously excluded. He is indeed free of the corruption of the Old Gods, but he is pragmatically deadly, and employs you in the elimination of the very few remaining Black Dragons in the world. When all is said and done, he has succeeded, and while Baron Sablemane remains an open question, Wrathion at the very least clears Azeroth of his kin.
Visions of Destruction:
Wrathion is only a couple years old when the Alliance/Horde war tumbles over the land of Pandaria. The conflict has exploded into a fully-fledged war, and it appears it won't end without one conquering the other.
Wrathion travels to Pandaria and enlists heroes of all stripes (not just Rogues) to try to push the war to a swift conclusion. He has had a vision, he presumes from the Titan technology that birthed him, of the Burning Legion's invasion. Wrathion wants the war ended swiftly and decisively, and for a new united front across Azeroth to prepare for the arrival of this invasion.
Initially, Wrathion favors the Horde, given Garrosh's aggressive policies and clear will to conquer. But as it becomes clear that Garrosh is dividing his own faction, and could never bring Azeroth's people together in any cohesive whole, Wrathion switches to favoring the Alliance.
But when Varian allows Vol'jin to ascend as Warchief and the two factions agree to a peace treaty, Wrathion is enraged. He is convinced that only a total absorption of the Horde into the Alliance would be enough to face off against the Legion.
Here's where things start to get a little tin-foil-hatty. Garrosh is placed under arrest, awaiting his punishment in Pandaria (which I believe was execution, though I could be wrong. Pandaren don't seem like the types to be into capital punishment.)
Meanwhile, the Bronze dragon Kairoz has been studying the strange effects of the Timeless Isle. Following the Cataclysm, the with all the dragonflights effectively de-powered into mere mortals, the Bronze flight's insight into the flow of time has been lost, and Kairoz is trying to get it back.
So Wrathion, still pissed at Varian, gathers some allies to break Garrosh out of jail. He gets Zaela and other Hellscream loyalists to assist, as well as Kairoz, and then several Infinite Dragons. Yes, the Infinite Dragonflight was part of Garrosh's escape.
Kairoz brings Garrosh and the others to an alternate timeline of Draenor - one that is 35 years in the past, and is slightly different in a few subtle ways. Wrathion comes with them, but he becomes separated at some point. The Infinite Dragons are nowhere to be seen.
That is, unless you count Kairoz. In truth, he never displays the Infinite corruption we've seen in other dragons, but this seems like a very likely first step along the way to the corruption of the Bronze Flight - something we already know about because the Infinites have been traveling back to our past to try to change the timeline.
The goal here is to have Garrosh create a new Horde that will conquer Azeroth. Wrathion wants it to do so in order to provide a strong war machine to fight off the Legion. Kairoz wants it so that he can have a potentially infinite army under his control, hopping from universe to universe and replicating his forces.
Garrosh just wants revenge. And once Kairoz makes it clear that this would only be an Orc-led Horde officially, but not in practice, Garrosh murders him.
Garrosh meets with the alternate-universe doppelgänger of his father and appears as a prophet, showing him a very distorted vision of the Horde's history - focusing a lot on the drinking of demon blood and then the internment camps - much less Thrall's successful bid of independence or Garrosh's own making a muck of it.
The Iron Horde is created, and Gul'dan is imprisoned - but not killed. Not content to simply have his ideal all-Orc Horde, Garrosh wants to return to Azeroth and take revenge. This, of course, bring the attention of the Alliance and the Horde proper, who very swiftly dismantle the Iron Horde. With Garrosh and most of the Iron Horde's leadership dead, and Gul'dan released during the initial fighting, the remaining leadership of the Iron Horde deposes Grom and instead turns to Gul'dan - ironically creating the exact "failed Horde" Garrosh had come to prevent.
This new Gul'dan slips through our grasp in the attempt to take down the Fel Iron Horde, and he eventually travels to our Azeroth, with thirty years of history and knowledge of his own doppelganget's mistakes.
It's here that the details become fuzzy, and I'm sure we'll have to wait until Legion to really find out what's going on. But Gul'dan finds the body of Illidan Stormrage, recovered from Outland several years ago and sealed within a crystal prison in the Broken Isles.
Somehow, Gul'dan is able to use Illidan to open a massive portal at the Tomb of Sargeras that allows the Legion to invade our world.
This is a story of compounded irony. Wrathion seeks to save the world from the Burning Legion, and through his actions, he creates the opportunity for them to strike. Garrosh seeks to create a pure Horde, free of the history of demonic corruption, only to have the Iron Horde fall to the demons and leave Azeroth threatened by an even greater Legion invasion than the Third War.
But if you really want to trace it all the way back, we can blame ourselves. We were the ones who messed with a bunch of Black Dragon organic matter in an attempt to create something that hadn't existed in ten thousand years. You can hardly say it was intentional, but we played our own part in bringing this whole mess to pass.
So I guess it's our job to clean it up.