Hearthpwn

With the release of Descent of Dragons, we saw the return of the original 5 "Dragon Aspects".  Previously all featured in the Classic set, each Dragon was given another powerful card in the latest expansion.  

Alexstrasza, Malygos, Deathwing, Nozdormu, and Ysera -- originally proto-dragons -- first combined their powers in an effort to defeat the "Father of Dragons", Galakrond.  With the help of the titans, their quest to end Galakrond's evil ways was successful.  Their show of power led to the titans naming the Dragons leaders of the Dragonflights, and primary protectors of Azeroth.  They represented the red, blue, black, bronze, and green Dragonflights respectively.

Their time overseeing the well-being of Azeroth saw the death of 3 of the original Aspects.  Malygos saw his demise after blaming mortal spellcasters for all of Azeroth's hardships.  He was defeated by Alexstrazsa's red dragonflight and allies.  Deathwing was killed in a combined effort from the Alliance, Horde, and other Dragon Aspects after being driven mad by the Old Gods.  The final Aspect to see their death, Ysera, fell victim to the Nightmare.  She was put to rest by Tyrande Whisperwind.

Quote from Alexstrasza »

"The champions who fought at our side assured the survival of our world. But now, we must see it with mortal eyes. We Dragon Aspects have fulfilled our great purpose, and our ancient power is expended. But though our day draws to an end, life endures and new generations will be born. Today's victory belongs to all who stood against the shadow. You are Azeroth's true guardians and the future of this world is in your hands. For the dawning of the age of mortals has begun." 

Alexstrasza

Dragonqueen Alexstrasza -- the "Life-Binder" -- is the leader of the red Dragonflight. She was empowered as the guardian of all life within Azeroth and deemed the "Queen of all Dragons". She led the crusade in the defeat of the maddened Deathwing, defeating him with her Dragonflight and her fellow Aspects.

Malygos

Malygos, Aspect of Magic -- the "Spell-Weaver" -- was the leader of the blue Dragonflight. Known as the keeper of arcane magic, he eventually came to believe that mortal spell-casters were causing the troubles of Azeroth. He was put to rest by Alexstrasza and the remaining Aspects.

Deathwing

Deathwing, Mad Aspect -- the former "Earth Warder" -- once Neltharion, he was driven to madness by the Old Gods. The newly titled "Aspect of Death" became consumed in defeating both the Alliance and Alexstrasza. The Aspects would go on to defeat him with the help of Thrall and an ancient artifact, "The Dragon Soul".

Nozdormu

Nozdormu the Timeless -- the "Timeless One" -- the leader of the bronze Dragonflight. The most mysterious of the Aspects, Nozdormu often remains in seclusion. His future, "evil", counterpart is Murozond the Infinite.

Ysera

Ysera, Unleashed -- the "Awakened" -- the original leader of the green Dragonflight. Once bound to the ethereal realm known as the "Emerald Dream", she would originally ward off the "Emerald Nightmare" to awaken; though she lacked the ability to differentiate dreams and reality. However, she would eventually fall to the aforementioned Nightmare and sadly be put down by Tyrande Whisperwind.