Deathwing

Ancient History 

Thousands of years before the First War the Dragon Aspects were created, by the Titans,  from the children of the massive Protodrake Galakrond. There were five children that were birthed and one of them was Neltharion, the Earth Warder. He was given charge over the earth and the deep reaches of Azeroth by the great Titans who cleansed Azeroth. In his early years as the Aspect of the Earth he was known to be very wise and a stalwart leader among all the other Aspects. Like many great things Neltharion also had his downfall. The Old Gods of Azeroth began to torment the great dragon  through his thoughts and dreams. They had utterly convinced Neltharion that he could have power beyond his wildest imagination.
 
The Old Gods convinced him to make the Dragon Soul, which would later become known as the Demon Soul after his transformation into Deathwing the Destroyer. With the help of the Goblins he then forged this Dragon Soul in the fires and cauldrons of the Goblins. The Dragon Soul turned out to be a small golden disk, but it was imbued with Neltharion's Magic and the taint of the Old Gods. With the constant whisperings of the Old Gods Neltharion began to get very paranoid about his own Dragonflight. He began to want a world just for him and his Black Dragonflight. Except Alexstraza and Ysera, he wanted to keep them around as some kind of Draconic sex slaves essentially.
 
Meanwhile in another part of the world Highbourne Elves led by Queen Azshara were consorting with demons.  The Burning Legion was starting to sneak into the earth once again, in preparation for their master Sargeras to enter Azeroth. Neltharion convinced all the other Dragon Aspects to give a bit of their essences to a super weapon to defeat the legion.  This was the Dragon's Soul. It was after this betrayal that his name changed from Neltharion to Deathwing the Destroyer.   
 
Except of course he cheated.  He put all of the other Dragon Aspects essence's but his own into the Dragon's Souls and betrayed all the good guys.  Many of the other dragon aspects had to go into hiding and thus begun the Dragon war. When all of the essence's were imbued into the Dragon Soul along with the magic of Deathwing, but not his essence they marched upon the the Burning Legions Portal and unleashed his great power destroying thousands of Demons, but also allies in the fight against the Burning legion. He was at last revealed to be a traitor, and with this the Blue Dragonflight surrounded him, but with one swift stroke of the Dragon soul he nearly annihilated the Blue Flight. This happened because of the arrogance of Malygos, he thought he and his Dragonflight could take on Deathwing, but he was sorely mistaken when Deathwing nearly wiped out the entire Blue Dragonflight.  Incidentally this isolation was part of what led to Malygos' turning on everyone else and having to be destroyed at the end of the Nexus war. Malygos was thrown into a deep downward spiral about the grief he felt because of his lost Dragonflight. 
 
This was a great blow dealt to the other Aspects, they began to get very paranoid about the power Deathwing had in his possession. So all of the Flights went into hiding and shielded themselves from Deathwing and even each other.
 

Transformation into the Destroyer

After he had completely been consumed by the taint of the Old Gods, his perfect Draconian from began to change. his proximity to the Dragon Soul and it power began to literally tear his body apart. Cracks began to from on his body, revealing his molten heart and his magma born blood. Fire and Magma began pouring out of his chest. His eyes became aflame in red, this tells how far the once wise and powerful had fallen to the corruption and evil of the Old gods. 
 
After his demonstration of  Power unto the mortal world, he retreated back to his mountain hideout. The longer he spent with the Dragon Soul the more his body began to tear apart, so Deathwing had the Goblins forge him the only thing that could hold his body together, adamantium plate armor. The Old Gods true intentions were for the Demons to discover and use the Dragon Souls power to bring their Lord Sargeras into Azeroth via the Well of Eternity. In the process of the portal being opened, the three Old Gods hoped to escape from the prison they were put in by the Titans thousands of years ago.
 
The plan panned out well for the Old Gods, a young Night Elf Druid by the name of Malfurion Stormrage stole the Dragon Soul using the Emerald Deam. When he awoke from the dream he was then captured by his twin brother Illidian Stormrage, who was working for the Burning Legion because of the promised made to him. The Burning Legion then used the Dragon Soul to make the portal even larger, but one thing they did not expect is the anger of Deathwing. he then stormed to retrieve the Dragon Soul. he released all of his anger and rage and succeeded in retrieving it for a short time. He was wounded badly in his attempt to retrieve it, but it was in vain. After he got the disk an gigantic power surge rocked Azeroth and Deathwing lost his grip on the Dragon Soul and he was forced to retreat back into his mountain kingdom.
 
After the Sundering of Azeroth the Dragon soul was recovered by the other Aspects of Azeroth and was imbued with an enchantment that kept Deathwing from ever using this powerful object ever again. The Dragon soul was then hidden by Malfurion Stormrage in a very secret place.
 
As Neltharion, he had been the Earth Warder, he had protected the many races of Azeroth from war and each other by raising the mountains high above the sky. After his fall from grace he was then said to lower these mountains and allowed the races to intermingle with each other. He also ruined the food sources and the resources of many races just out of pent up rage and anger.
 

Deathwing  in the Second War

    Deathwing's plotting didn't end with the War of the Ancients.  He was important in the Second War as well.  He shows up as Lord Prestor in Alterac.  He was infiltrating the Northern Alliance trying to bring it down from the inside.   
 
Meanwhile he also had plans on defeating the other Dragon flights.  Deathwing gave the Demons' soul to the Dragonmaw Orcs and they red Red Drakes.  They kept Alexstraza in Grim Batol while she gave them eggs for them to ride.  When that plan failed Deathwing took several eggs through the Dark Portal into Outland.   
  

Deathwing in World of Warcraft

Deathwing has laid silent for most of WoW.  His two children Onyxia and Nefarian had plans to wreak havoc but both were stopped by some combination of Varian Wynn, Bolvar Fordragon and between 10-40 adventurers.     

In one quest in Burning Crusade it was revealed that Deathwing still lives according to his Prime Consort Sintharia.   Furthermore the eggs he brought through the dark portal were warped by the twisting nether and some of them became what eventually developed into the Netherwing flight of Dragons and Drakes.   

Deathwing is the main antagonist of Cataclysm, as Arthas was for Wrath.
 
He is also the father of Nefarian and Onyxia.