"You mortals are nothing to my kind! Do you hear me? Nothing!"
Nefarian was a black dragon, the son of Deathwing, mad scientist, and Lord of Blackrock Spire. From the top of the Spire, he commanded the Black Dragonflight, and the Dark Horde. While his chosen commanders,
General Drakkisath, and
Warchief Rend Blackhand, led his forces against the Dark Iron Dwarves and the minions of Ragnaros, Nefarian spent most of his time in scientific experimentation.
In his scientific endeavours, Nefarian sought to create a new dragonflight, one under his command, of course, by combining the strengths and powers of all five dragonflights. His success was mixed, as many proved unstable, however he was able to produce some viable specimen, such as Gyth, the chromatic drake that served as Rend Blackhand's mount.
After successfully corrupting the red dragon,
Vaelastrasz, he claimed the Red Dragonflight's piece of the Sceptre of the Shifting Sands. He then attempted to destroy it. Had he been successful, he would have likely doomed Kalimdor to being overrun by the silithid.
Adventurers, having fought their way up the spire, defeating Rend, Drakisath and his other minions along the way, finally engaged him in battle. By turning their own powers against them, he was able to defeat the adventurers, and drive them out of his lair. He was, however, killed when he attempted to devour a
Toyota Tacoma while visiting Azshara.
"I have tried to be an accommodating host, but you simply will not die! Time to throw all pretence aside and just... KILL YOU ALL!"
Having a pickup truck burst through his chest would not be the end of Nefarian, however. Deathwing, soon after his return to Azeroth, raised Nefarian from the dead. Seeing the value in his experiments, Deathwing tasked Nefarian to renew his efforts and continue them. To that end, Nefarian returned to Blackrock Mountain, and established a new lair and laboratory for himself there. His new lair, Blackwing Descent, was hidden much deeper within the mountain than his old lair. Nefarian continued his experiments, however he also developed a new fascination with undeath, possibly as a result of his own newly acquired undead state. From these experiements, he brought his sister, Onyxia, back to life, albeit as some sort of mindless electrical powered zombie dragon.
Despite his defences, minions, monsters, and all his snark, adventurers stormed Blackwing Descent and killed him a second time.