Originally, Deathwing was simply going to use the Demon Soul to control all of dragonkind. This backfired when Malfurion Stormrage stole the Demon Soul during the War of the Ancients. The Demon Soul was hidden away, as nobody could figure out how to destroy the thing -- it was impervious to all damage. And so the Aspects put their own spell on it -- because it could not be destroyed, while it existed Deathwing would be unable to use the thing for himself. Terribly clever thing for the Aspects to do, but cleverness begets cleverness, and so Deathwing had to think of another plan.
Since Deathwing couldn't simply control the dragons anymore, the only option left was to murder every last one of them, starting with finishing off that pesky blue dragon flight up in Northrend. And so, while Deathwing went into hiding, the black dragons of Azeroth waged a terrible war against the blues, as well as the other flights. They attacked the Dragon Riders of Loreth'Aran, who had been blessed by Ysera herself and allowed to ride the green drakes of Ysera's brood, destroying all of them and leaving the spirit of their leader to roam the ruins of his city and tell his story. It is said that the black dragons also attempted to fight the bronze dragons, but they didn't succeed -- mainly because the bronze flight didn't hang around and fight as expected, instead scattering across the world and going into hiding.
But Deathwing wasn't in hiding per se -- he was plotting. With the other races out of the way, he'd have his black flight to take over the world, but the black flight was notorious for fighting not only with the other flights of dragons, but with themselves. The overtaking of Azeroth required a united effort, and Deathwing knew this -- and so he hatched another plan: to breed his own flight of dragons, wholly devoted to him and carrying out his purposes. His next plan of attack was deliberate -- if he could not breed his flight on Azeroth, the world of Draenor, accessible through the Dark Portal, would suit his purposes nicely. As a bonus, he'd have a second world under his watchful claws.
In the meantime, two of his most devoted children, Nefarian and Onyxia, worked on Azeroth to not only infiltrate the pesky mortal races that had been such a thorn in his side, but to work on their father's unique breeding program -- Nefarian going so far as to originate the Chromatic dragonflight. This too, ultimately failed, and both Nefarian and Onyxia met a swift end at the hands of those pesky mortals that Deathwing just couldn't seem to get rid of. Not only that, but the mortals managed to grab a foothold on Draenor -- enough of a foothold that the majority of the precious eggs he'd transported were destroyed.
Deathwing returned to Azeroth, and the eggs left behind on Draenor had a curious fate of their own
-- warped by the energies of the Twisting Nether that were released upon the destruction of Draenor and the forming of the shattered continents of Outland, his children changed, twisting into draconic creatures that almost seemed to be half composed of the nether itself. While the Netherwing were never directly under the black flight's control, they could not be ignored -- and so, Deathwing continued to work in his own curiously manipulative way.
While Deathwing could not use the Demon Soul, he could manipulate others into using it for him, which is what ultimately led to Alexstrasza's kidnapping
at the hands of the Dragonmaw clan of orcs. But Alexstrasza was never his target. He was after the eggs that she'd been forced to breed, to use for his own purposes. When this plan ultimately failed with the destruction of the Demon Soul at the hands of Rhonin, Deathwing was once more forced to go back into hiding and regroup.
Deathwing's prime consort, Sintharia, had a plan of her 'own design' -- to continue her son Nefarian's failed experiments and engineer a race of dragons that would quickly overtake Azeroth, a new breed of dragon flight that would bow to her every whim. But how would she control these new, deadly dragons? The answer to that was very simple -- by rebuilding the Demon Soul. The Demon Soul was destroyed by Rhonin
back in Day of the Dragon, when Alexstrasza was rescued, but the shards still existed. With a little engineering, Sintharia managed to put the shards of the Demon Soul back together, and ward it against further tampering by making certain that no power born of Azeroth could shatter it -- including any black dragon scales.
So Sintharia had her method of control, the next item on the list was how exactly to engineer these drakes to make them more powerful than any other flight? Well, the Netherwing had made a presence in Outland, and they were already extremely powerful. Black dragons that had been infused with the essence of the Twisting Nether, their energies would provide the extra oomph that Nefarian's Chromatic dragons were missing. So Sintharia traveled to Netherwing Ledge and made a deal with the orcs -- trading the services of the black flight for netherwing eggs, and letting it slip that 'her master' could use them.
The notice that Deathwing was alive was more than enough to make Overlord Mor'ghor agree to Sintharia's request. But what Lord Mor'ghor didn't know is that Sintharia wasn't really working for Deathwing at all. Sintharia had the unfortunate experience of surviving mating with Deathwing -- a creature largely composed of molten lava. The experience left her with half of her face literally melted away, and the scars on her face and body continued to burn, thousands of years after Deathwing had supposedly disappeared -- more than enough to drive anyone completely insane.
As far as Sintharia was concerned, the black flight was dead, along with Deathwing -- and she could care less. Why should she care, when she could bring into the world a far more worthy flight of dragons who truly would become gods? Besides that, there were voices speaking to her while she slept that assured her this was a very good idea. And as we all know, having voices speaking in your head and telling you about wonderful ideas is a fantastic
thing and sure to end well. Right.