Meanwhile, even after the Old Horde finally met its defeat, Deathwing took part in other gambits. He approached the forces of the former shaman Ner'zhul with a bargain, granting his assistance and that of his offspring to the Horde of Draenor in exchange for safe passage through the Dark Portal for himself and clutches of black dragon eggs. After helping Ner'zhul's lieutenants Grom Hellscream and especially Teron Gorefiend in claiming several of the artifacts Ner'zhul would need for his plan to create many other magical portals in order for the Horde to escape Draenor's slow, rotting death, the Aspect of Death demanded Ner'zhul turn over the Skull of Gul'dan to him.
Ner'zhul either didn't feel he needed the skull any longer, or wisely didn't want to get into a fight with Xaxas the Black Scourge (I love Deathwing's many, many titles) and handed his former apprentice's demon-tainted cranium over. It remained in Deathwing's possession until the combined forces of Gruul the Dragonkiller and the Alliance Expedition heroes Khadgar, Alleria and Turalyon bearded the ancient dragon in his den in the Blade's Edge Mountains. After a tremendous battle where Deathwing showed Gruul what true power was by nearly ripping the Gronn patriarch in half, only to retreat when Khadgar managed to use his magic against the adamantite holding the Aspect of Death together. Deathwing fled, and the Skull of Guldan was used by Khadgar to seal the Dark Portal and protect Azeroth when Ner'zhul enacted his great ritual and in so doing, tore Draenor asunder.
We don't know why Deathwing wanted the Skull of Guldan. We do know, however, that before Khadgar had it in his possession Deathwing, the creator of the Demon Soul (an artifact so powerful that both Sargeras and the Old Gods believed it could be used to open portals to allow them into or out of Azeroth) had it and presumably studied it. It is extremely unlikely that Deathwing didn't understand exactly what Ner'zhul was about to do, especially when he helped Gorefiend collect several of the artifacts for the ritual, since Deathwing was far, far more experienced with arcane magic than Ner'zhul was.
The screenshot above is an image of Deepholm, the realm of earth on the elemental plane connected by the Titans to Azeroth. For contast, here is an image of the Netherstorm region of Outland.
There's more to consider, however. The creation of the Netherwing led both to the revitalization of Malygos (one of his blue dragon servants, Tyragosa, used nether dragon "essence" to supposedly cure him of his madness that dated back to Deathwing's initial betrayal and near total destruction of Malygos and his blue flight) and was essential in both Lady Sintharia's work and the later use of that work by Deathwing to create the Twilight Dragonflight. In essence, without the destruction of Draenor, there would be no Twilight Dragons now.
So what we have here is a grim possibility. Deathwing always intended the destruction of Draenor, and he manipulated the primary figures of the Horde and even the Alliance Expedition to bring it about. He manipulated the orcs into enslaving Alexstrasza and the reds, then used that connection to move himself and some of his eggs to Draenor, where he could study what Ner'zhul was planning and ensure it would help create a perfect test bed for what he intended to do back home. Why else would he choose to go to Draenor in the first place? To raise his offspring in the very midst of the cruel and sadistic gronn? Clearly, he could not have chosen a worse spot if his goal had ever been their safety. But as a nightmare crucible to forge an entirely new species of dragon, Blade's Edge was ideal. It allowed him to not only leave his eggs in a place he knew would be bathed in the fury of what was to come, it also allowed him to study the gronn and their ogre subjects, a study that would serve him well when the Twilight's Hammer under Cho'gall would come to serve him.
Another possible clue to all this is when you take part in the battle against Yogg-Saron in Ulduar. In that battle, Yogg-Saron's very brain is the battleground at times, and in said brain a series of moments in Azeroth's history unfold. One is the death of king Llane Wrynn, his heart cut out by Garona due to the powerful magics of Gul'dan and Cho'gall and the other warlocks of the Shadow Council. Another is the creation of the Demon Soul. When one considers that Yogg-Saron calls himself the Old God of Death and that it is only after his destruction that Deathwing returns as the self proclaimed Aspect of Death and that Loken, prime designate of the Titanic Watchers fell to Old God corruption in much the same way that Deathwing did, we start to see a pattern. The Old Gods have tried to use portals originally summoned for the Burning Legion before. We know that an Old God was being summoned into Shadowmoon Valley, the very heart of Gul'dan and the Shadow Council's power, by Arakkoa who ended up destroyed. What we have in the brain of Yogg-Saron is the Old God of Death taking credit for everything.
We therefore have a series of events that leads from the Old Gods to their sometime ally, sometime usurper Deathwing (who dares to claim powers that they themselves also claim) and his machinations against both the Alliance and the Horde, using each as pawns and catspaws when it suits him. Acting through a goblin proxy, he manipulates members of the Horde into not only neutralizing one of his greatest enemies (and heaping a continual humiliation upon her and her consort that leads to that consort's death), but in so doing, learns enough about them to then use them again to help bring the very artifacts they intend to use to escape their doomed world into his grasp. He then allows a human mage to take one of the artifacts from him by feigning defeat, just as he does when he re-enters Azeroth from the Dark Portal so that he can assume the Daval Prestor identity. He leaves his eggs behind to be exposed to the destruction he always intended, an accelerated test bed for what he has planned for Azeroth.
The destruction of Draenor and creation of Outland become one more stage in Deathwing's ultimate plan to reshape Azeroth to his liking. He knows now exactly how far he can bend a world before it breaks ... and exactly what kind of breaks will bring about the conditions he desires. Outland serves as a primer in how to undo the works of the Titans, and whether or not Deathwing is acting of his own accord or is still under the manipulations of the Old Gods, it's clear that the Aspect of Death plays a long game, and the mortal races need to start trying to get ahead of him.
OK, thanks for putting up with my crazy. Next time, Yogg-Saron, I promise.