My master... He continues the work that his progeny began.Though the Black Dragonflight is in the process of dying out, that isn't stopping Deathwing from trying to keep it alive. Sort of. Over the course of Warcraft, Deathwing has been on a very deliberate mission to repopulate the world with dragons of his choosing. During the Second War, Deathwing discovered the location of the Demon Soul, a powerful artifact he created back during the War of the Ancients in order to control the other dragonflights.
Deathwing wasn't able to wield the Demon Soul, however -- the other Aspects placed a powerful enchantment on the device so that he would no longer be able to use it. But Deathwing was a very clever dragon and realized this meant he simply had to find someone else to use it in his stead. Through visions, he led a powerful orc from the Dragonmaw clan named Zuluhed the Whacked to the artifact. Zuluhed couldn't decipher how to use the thing, and so he handed it over to his second in command, Nekros Skullcrusher. Nekros then promptly used the thing to enslave Alexstrasza the Dragonqueen.