"Scream your dying breath!"
Wait. It's Kel'thuzad
and his quote isn't "Your curiosity will be the death of you."
Kel'thuzad was once a human mage and a member of the Kirin Tor. However, having become fascinated by necromancy after witnessing its use by the death knights employed by the Orcish Horde during the Second War, he was quick to cast aside those allegiances, after all, the Kirin Tor's shortsightedness and cowardice were just holding him back, and journey to Northrend, heeding the call of the Lich King. In return for his loyalty, the Lich King promised Kel'thuzad power and immortality. Kel'thuzad gladly accepted.
Returning to Lordaeron, Kel'thuzad established the Cult of the Damned, and achieved his life long goal of starting his own religion. He also founded his own school of necromancy, Scholomance, secreted away under the town of Caer Darrow. Through the Cult, Kel'thuzad soon began to spread the Plague of Undeath throughout Lordaeron. He was then killed by Prince Arthas as part of the Lich King's plan. While some may have seen that as a betrayal, Kel'thuzad understood that it was merely part of a greater scheme. His loyalty was soon rewarded, as Arthas returned from Northrend as death knight and loyal servant of the Scourge, and was tasked with raising Kel'thuzad from the dead. Using the power of the High Elves' Sun Well, and corrupting it in the process, Kel'thuzad ascended to lichdom. Together, Arthas and Kel'thuzad carried out the will of the Scourge's demonic masters, and summoned Archimonde into Azeroth.
The Scourge was left to its own devices in the wake of the Burning Legion's defeat. Kel'thuzad served as majordomo to King Arthas for a time, but the death knight was soon called back to Northrend by the Lich King, leaving Kel'thuzad in charge of the Scourge's holdings in Lordaeron. Commanding the Scourge of Lordaeron from the floating Necropolis, Naxxramas, high above the still burning ruins of Stratholme, Kel'thazad seemed to be the indisputable ruler of eastern Lordaeron. A land now known as the Plaguelands. However, there were still holdouts against the Scourge, and Kel'thuzad was ultimately defeated by the joint efforts of the Argent Dawn and the Scarlet Crusade. Tragically, while his phylactery had been captured, the man tasked with delivering it to the Argent Dawn for its speedy destruction, Father Inigo Montoy
, never did. It was soon discovered that Montoy had been a Scourge agent all along, and had returned the phylactery to his masters. Naxxramas, however, was seemingly forced to retreat, leaving the skies above the Plaguelands and returning to Northrend.
As the forces of the Alliance and Horde began their assault on Northrend, Naxxramas was quick to counter attack, launching an assault on the Alliance fortress of Wintergarde. The dread citadel was soon under renewed attack by large groups of adventurers, this time coming at them in groups of ten or twenty five, rather than forty. While Kel'thuzad and Naxxramas were likely defeated, it seems that his phylactery managed to elude destruction once more.
I kind of suspect that Kel'thuzad was originally planned to be part Icecrown raid, but Blizzard had been getting some flak for recycling content during Wrath of the Lich King, so I think they nixed that in favour of having a new raid lich encounter.
Anyway, it would appear that Kel'thuzad managed to survive the fall of the Lich King. However, with the newly appointed Lich King Bolvar sitting on the Frozen Throne still in a state of dormancy, there's not much for an Archlich and massive flying fortress to do. Having revived the Cult of the Damned as the KT Fanclub by offering miniaturized versions of himself to members
, selling the use of his likeness and those of his minions in Naxxramas to a popular card game and even going so far as to convert Naxxramas into a massive cardsoleum, Kel'thuzad has at least been keeping busy.