Deathwing

"Pain... agony... my hatred burns through the cavernous deeps."

Deathwing the Destroyer is the Aspect of the Black Dragonflight and the most powerful dragon ever to live. He works for the Old Gods because he believes their rule is inevitable and that there is nothing he can do to stop it. That revelation eventually drove him mad.

Deathwing is the one to set in motion the Apocalypse which is the setting for the majority of the second half of the series.

History
Neltharion the Earthwarder, or Neltharion the Black, was among the five great dragons tasked by the Titans with keeping order on Azeroth. Neltharion's task was to manage the deep places of the world, in the mountains and caverns. Eventually Neltharion started hearing whispers in his head urging on his subconscious impulses that may not have been the most ideal ones. This eventually grew into the whispers outright urging him to commit atrocities he never before would have considered. However, the dark whispers over time convinced him and this is when he began to lose his grip on his sanity.

When the Burning Legion first invaded, Neltharion had convinced the other four dragon Aspects to lend their essences to a device contrived by the whispers, known as the Dragonsoul. The purpose of the device was to drive out the demonic invaders... or so the other Aspects thought. Neltharion, now known as Deathwing, instead used it to decimate the other flights, as well as declare his dominance on the world. Eventually the power from his own artifact began to shred his body apart. During this state, the other Aspects stole the Dragonsoul from him and hid it away as he retreated into the depths of the planet to recuperate.

Roughly ten thousand years later, the location of the Dragonsoul was revealed. Deathwing was sent by his masters to retrieve it, using an alliance with the Horde (at that time controlled by the Burning Legion, who had recently returned) during the Second War, and managed to use it to enslave Alexstrasza and attempt to reinstate his dominion over Azeroth. However, his plans were foiled when the Kirin Tor was able to restore the Dragon Aspects' essences from the Dragonsoul by destroying it. He could not take on all four other Dragon Aspects at once with their renewed power, and retreated into the cavernous depths once again.

Ever since then, however, his children have been carrying out experiments in order to realize their father's dream: enforce the dominance of the Black Dragonflight over the world. Not even they realized, however, that Deathwing's ambitions were far deeper than that. Having spent his time in the Elemental Plane licking his wounds caused from continued usage of the Dragonsoul's dark powers, the whispers in Deathwing's head have only gotten more maddening. The whisperers have bestowed in him power beyond his imagining, forging him into their harbinger of the apocalypse. And when the day comes, Deathwing the Destroyer will emerge from beneath the planet and carry out his masters' will.