The genesis of the Netherwing is an odd story. Deathwing the Destroyer, the by-now insane leader of the black flight had made a deal with Teron Gorefiend shortly after the orcs were defeated in the Second War. Gorefiend had been ordered by Ner’zhul to go to Azeroth and retrieve several artifacts of power that Ner’zhul needed to open dimensional gateways to other worlds. By doing this, Ner’zhul hoped to escape Draenor, and the influence of the Burning Legion, for good — and take his people to another land in which they could begin to rebuild after the staggering losses from the wars, and the orcs corruption.
The gist of Deathwing’s deal was this — Deathwing and his black drakes would help Gorefiend find the artifacts he was looking for. In exchange, Deathwing wanted access to the Dark Portal so that he, some of his drakes, and some cargo he had could be transported to Draenor. They would continue assisting the orcs on the other side of the portal as long as the orcs left the drakes, and the cargo, alone. While Gorefiend wasn’t sure he trusted Deathwing’s motives entirely, the addition of drakes to their fighting forces would make things much, much easier — and so he agreed.
The items that Deathwing wanted transported, of course, were dragon eggs — eggs that Deathwing had gathered for the express purpose of breeding his own flight of dragons that would bow to none other but him. Why Draenor? Well, why settle for one world when you can have two of them? Deathwing wanted total domination over Azeroth, and domination over Draenor as well was just icing on the cake. And so Deathwing, his troops and his cargo traveled through the Dark Portal and into Draenor, where they met with Ner’zhul. Deathwing asked Ner’zhul for one more thing – the Skull of Gul’dan, a powerful magical artifact. While Ner’zhul didn’t really want to part with it, he didn’t exactly need it anymore, and so he handed it over. Deathwing left some of his troops behind with the orcs, and took the large chunk of his forces to Blade’s Edge, along with the eggs. (Via Wow.com)