We’ve forgotten (or have never explored) the story before WarCraft III when the game was much simpler than it is now in World of Warcraft and the lore was confusing yet simple. Before the time of Arthas there were two epic wars between Humans and the allied nations of the Alliance and the Orcs and the vicious Horde. There was constant strife and brutal battles where armies would sortie and both sides would be decimated. Yet, at the same time, other great evils lurked within the world. One was known as Neltharion, the Earth-Warder one of the five great Dragon Aspects and leader of the black dragonflight.

Long before the War of the Ancients, when the titans visited Azeroth, they empowered  five dragons with various powers. Neltharion was given the dominion over the earth and was set to protect it with his brood. Yet, they also imprisoned the evil Old Gods within the ground, Neltharion’s dominion, which eventually began corrupting him, creating what we now know as Deathwing. During the War of the Ancients he betrayed his fellow dragons and manipulated them to create the Dragon Soul, a weapon containing the power of every dragon aspect except for Neltharion, which he then kept to himself. The dragons came to retrieve it, but they were facing their own stolen power along with Neltharion’s Old God corruption.

Deathwing is one meaning looking dragon now.

With a mighty swing of the Dragon’s Soul he crushed the majority of the Blue Dragonflight, leading to Malygos’s madness (he was a great asset in creating the weapon of which Deathwing wielded). Then, he continued to destroy the various aspect’s flight until such a point that Malfurion used the Emerald Dream to gank the soul back, leading back to the story of Illidan. Essentially it is taken, hidden, and enchanted so that Deathwing could never find it nor use it.

After that event, Deathwing swore to destroy everything, and disappeared for 10,000 years. It wasn’t until the Second War that he reappeared and was defeated by mages sent by Dalaran. His corpse flew into the ocean, but we now know he was not killed. He took up the human disguise of Lord Prestor. As Lord Prestor he worked hard to undermine the Alliance.

Of course, at the same time, he worked with the vicious Horde in order to make his way to Outland where he figured he could lay his flight’s eggs without relative harm. The Alliance Expedition found him and defeated him again, but not before he could flee back to Azeroth leaving his eggs behind. Those eggs became what we now know as Netherdrakes.

Returning back to the identity of Lord Prestor he began to pit the various kingdoms of the Alliance against one another. At the same time, he discovered the location of the Dragon’s Soul. Unable to touch it, he used the orcs and Zuluhed the Whacked to use the Dragon’s Soul for him. In his final act of lunacy, he tried to steal Alexstrasza’s eggs which were in Dun Algaz. Luckily, Rhonin discovered his plot and destroyed the Dragon’s Soul returning the power to the other Dragon Aspects. From there, he fled once more.

The cool thing is that Deathwing is really bitter because he has yet to do anything other than make Malygos almost as crazy as himself. The kind of not cool thing is that this sounds a lot like Arthas’ story where he keeps showing up, getting defeated, and blasting off again. So he’s full of rage, but we all have to hope he doesn’t show up at various instances, laugh at you, and then leave over and over until you kill him. After all, he’s demonstrated for us already how crazy he is.

Of course, we don't know yet the full reason why he's all of a sudden powerful enough to change all of Azeroth and become the biggest and baddest boss ever, but I'm sure all of that will be explained in the quest text. One thing many of us who follow the lore have to accept is that often the lore has to conform to the mechanics of the game (i.e. the need to have a single boss as the expansion's bad guy). WotLK brought the dragon aspects back from the few mentions of the Bronze flight and the books from the early 2000s, so we must assume that Deathwing will be continuing this in some weird and awesome way.

Lore Theorycrafting

Who could be the next main boss if it was not Deathwing?

Well, that’s a tough question. The only answer I could think of is the Burning Legion and possibly Kil'jaeden. There just isn’t much left on Azeroth outside of the Old Gods that poses a threat. That is, except for Azshara. Then again, who wants to fight a giant Naga after we had a TBC full of them?

I think the question pretty much sums up why Deathwing is an OK boss for the next expansion, even if he's yet to do anything at all other than fail. I mean, after all, even Arthas failed in the end. The main boss is going to need to be someone who can fail eventually, right?