If you remember Anne's excellent Elune is a naaru post from a few months back, then you'll know how this works. If not, then I'll explain. Basically, what follows is a huge post full of supposition and speculation that takes established lore (including some new lore from Cataclysm) and uses it as a framework to support a contention that is not currently stated by Blizzard. It's probably not what's actually going on, but like that guy you know who won't shut up about the grays, we're going to ride the crazy rocket all the way to violent crash landing town. (I promise you the Yogg-Saron post is coming, but in writing it, I ended up going down these roads for a bit.)
Deathwing. The destroyer, Blackwing Greatfather to the gronn of Outland, Xaxas to the night elves, formerly Neltharion the Earthwarder and still referred to among some of the earth elementals. The Dark One. Neltharion the Betrayer, Blood's Shadow, self-proclaimed Aspect of Death. The Unmaker of Worlds. Many are his titles, earned in acts of rampage and devastation from the time of the War of Ancients (when he used the Demon Soul to annihilate the Blue Dragonflight) right up to the Second War, when he helped the orcs make off with magical artifacts for Ner'zhul. Soon, the fury of Deathwing will be the focus of an entire story arc with the release of the Cataclysm expansion.
One of his titles, however, interested me. Deathwing now claims the mantle of death itself, which is interesting considering his relationship to the Old Gods, who as we know have a death god already. Furthermore, Deathwing now calls himself the Unmaker of Worlds. Now, we could just dismiss this as the typical grandiosity of a being whose pride and arrogance matches his power, but when you really start to think about it, it sounds much less like a boast and more like simple fact. Deathwing has indeed unmade a world. The preparations for this Cataclysm go back much further than we'd expected and have already borne stark fruit.
However, despite all this, Neltharion began to harbor doubts about the Titans and their creation. Why should dragons, the most magnificent of all their works, be trapped forever in the role of caretaker when they should instead be rulers? Who better than the dragons, after all, the pinnacle of form and the epitome of power? Furthermore, he looked with bland envy on his "sisters" Ysera and Alexstrasza, thinking that it was he who deserved such mates and not those who consorted with them. These doubts and lusts were fed by whispering voices as he communed with his charge, the darkest depths of the earth, and it may be that he never realized his mistake. For that matter, he probably doesn't consider it one.
Everything that comes after flows from this one moment where Neltharion stared into the abyss below the surface of the world that was his to guard and defend and blinked. His undeniable positive attributes of strength, eloquence and intelligence were twisted by his greed to possess what he was intended to safeguard and although he would not be immediately twisted visibly by them his corruption began at the exact moment he failed to turn away from the Old Gods. His eventual state as a being constantly torn apart from within by writhing molten fury, held together by adamantite plates constantly reattached to his body, was just a reflection of this moment.
The rest we've covered before. Neltharion, with the help of his best friend Malygos, convinced the other Aspects to loan him a portion of their power to create an artifact capable of standing off even the Burning Legion, the Dragon Soul which would become known as the Demon Soul. The War of Ancients, his betrayal of the other Aspects and destruction of most of the blue dragons (including Sindragosa, prime consort of his friend Malygos), the centuries long hunting of the few remaining blues by Deathwing and his Black Dragonflight ... down through the centuries Deathwing and his brood stood in opposition to the efforts of the other dragons.
While he was certainly a potent force for destruction and mayhem during the long period between the War of the Ancients and the War of the Shifting Sands, it's notable that Deathwing and the black dragons didn't play any role in supporting C'Thun and his war effort. Clearly, while Deathwing's current power lust and madness stems from his long contact with the Old Gods, he doesn't feel any particular loyalty to them or at least not to C'Thun.
No, the next time Deathwing moves forward as more than just a ravager would be when the mortal host of Sargeras breached the barrier between Azeroth and the red world of Draenor, allowing the corrupt, demon-blood-addled orcish hordes to pour through into Azeroth. Deathwing took one look at these guys and immediately decided they were his kind of scum.
He could use them. And use them he proceeded to do, with great alacrity and skill.