Kel'thuzad was one of the major villains of Blizzard Entertainment's Warcraft III: Reigns of Chaos, and a secondary villain in Warcraft III: The Frozen Throne. After his appearances in Warcraft III and its expansion, Kel'thuzad later returns in the next game in the series, World of Warcraft, as a major villain and final boss of the game. After a long absence, he returns in the game's second expansion pack, Wrath of the Lich King.
The Kirin Tor
Kel'thuzad was once a wise wizard of the Kirin Tor, a faction of the greatest spell casters in the world. He was a member of the Council of Six, the masters of the Kirin Tor. Of the Council, Kel'thuzad was the most eager to gain entry to the Library of the Guardian, and was the most disappointed upon learning of its disappearance. Eventually, Kel'thuzad came upon Necromancery, the forbidden magic of the dead. When the Council learned that Kel'thuzad had begun practicing Necromancery, he was exiled from the Kirin Tor capital city of Dalaran.
Soon after being exiled from Dalaran, Kel'thuzad began hearing voices. Strange voices... They drew him to the cold continent of Northrend, where he met the spider-like beings known as the Nerubians. The Nurubian king, Anub'arak, spoke with Kel'thuzad about his true master. Anub'arak was the unwilling servant and majordomo of the mysterious Lich King, an alien being who made his domain in Northrend and raised an army of the dead. Kel'thuzad was persuaded by the Lich King himself to join his cause, and was sent back to his homeland in the Eastern Kingdoms.
The Cult of the Damned
Kel'thuzad was given orders from his new master: To raise a cult of like-minded individuals who, too, sought to raise an army of undeath. After building his forces, Kel'thuzad had his cultists, disguised as traders, to hand out grain to villages so that the villagers could make bread. What the villagers did not know was that the grain was infected with the Plague of Undeath - an unnatural disease that turns people into mindless undead upon death from the plague. The Prince of the Kingdom of Lordaeron, Arthas Menithil, learned of this disease and tracked it down to the source. Arthas and Kel'thuzad battled, and the Necromancer was defeated... But not for long.
Several months had past since Kel'thuzad had been killed by Arthas. During those months, Arthas had led an attack on Northrend in an attempt to destroy the Undead Scourge's leader. Unfortunately, during his times in Northrend, Arthas came across a sword called Frostmourne, that slowly ate away at his very soul. Arthas went insane, and joined forces with the very monsters he swore to destroy. His orders from his new master, the Lich King, were to bring Kel'thuzad's decaying corpse to the mystical Sunwell - the source of all of the Elven magic. Kel'thuzad's body was dumped into the yellow waters of the Sunwell, thus corrupting it, forcing the surviving Elves to find a new source of power. Kel'thuzad had returned as a Lich - an evil, undead sorcerer that contains their soul in a jar called a Phylactery. Kel'thuzad, Arthas, and the undead Elf Sylvanas, ruled the northern Eastern Kingdoms for quite some time, until the Lich King's superiors, the demonic Dread Lords, arrived to take over.
Civil War of the Plaguelands
Months had gone by since the Dread Lords took over the Plaguelands. It was not long, however, until the Lich King gained his freedom and defected from the demonic Burning Legion. Unfortunately, the Lich King's powers were also damaged, with half of his army, including Lady Sylvanas, gaining their free will. A war broke out, and while Arthas was in Northrend to help the Lich King, Kel'thuzad was left to lead the Scourge against Sylvanas and the Dread Lords. Kel'thuzad's forces were quickly pushed back, and Kel'thuzad was forced to retreat.
Years later, the Plaguelands were mostly under the control of Sylvanas, with small parts under the control of the Scourge and the Argent Dawn, an army of Paladins and Priests. Kel'thuzad's "messenger boy," Baron Rivendare, was in control of the ruins of Stratholm, one of the largest of the human cities. A small army of heroes stormed the ruins, and killed the Baron. Not long after, Kel'thuzad returned and with him, the dreaded Naxxramas, a floating citadel of death. 40 unknown heroes stormed Naxxramas, slaying each of Kel'thuzad's top lieutenants and minions. A great battle took place in Kel'thuzad's throne room, resulting in his death. One of the heroes took his Phylactery, but instead of destroying it and killing Kel'thuzad for good, they handed it to a priest named Inigo Monty. Monty told the heroes he would see to it the Argent Dawn got the Phylactery... They never did.
War in the Frozen Wastes
Years later, it was revealed that Inigo was a follower of Kel'thuzad, and brought his Phylactery back to Northrend to resurrect him. Inigo was rewarded with undeath, and Kel'thuzad once again ruled Naxxramas in Northrend. Kel'thuzad was in charge of leading an assault on an enemy fortress called Warsong Hold. Kel'thuzad was pushed back, however, and returned to Naxxramas. A small army of heroes charged into Naxxramas once again, killing Kel'thuzad's top minions. In the end, Kel'thuzad was killed, but his Phylactery was nowhere to be found. His fate remains to be revealed...