He found a terribly good one in a human mage -- not just any human mage, but one of the Council of Six, the ruling high council of the Kirin Tor and one of the more influential and powerful mages in Dalaran, Kel'Thuzad
. Kel'Thuzad had been something of a maverick for years -- obsessed with learning everything there was to know about magic, Kel'Thuzad desperately wanted to study necromancy, something that was expressly forbidden by the Kirin Tor. When he heard the voice of the Lich King, he realized there was something far greater out there than Dalaran or the cowardly Kirin Tor that could teach him exactly what he needed to know, and so he packed a few things and left, leaving his fortune, his political standing, and his reputation behind. His destination? Northrend.
Upon reaching Northrend, he realized far too late the folly of his actions
-- the Lich King was not a benevolent being seeking to give him the knowledge he so desperately craved, but a soulless creature intent on wiping out all life on Azeroth. Left with no choice, he swore to serve the Lich King in all things, and was promised immortality and great power in return for his first assignment -- to go back to the lower continents and found a new religion, one that would worship the Lich King as a god. He left aboard the citadel of Naxxramas to fulfill the Lich King's wishes, and headed back south, to Lordaeron.
It didn't take long for him to acquire a substantial amount of followers, promising his acolytes eternal life on Azeroth in exchange for their devotion to the Lich King. The Cult of the Damned, as it came to be called, grew rapidly in numbers, keeping themselves secret for a few years while the Lich King prepared for his assault. With loyal followers from several villages, it was easy for Kel'Thuzad to distribute plague cauldrons, portable devices engineered to distribute the plague of the Lich King among the mortals that weren't so easily swayed.
The plague worked all too well -- citizens fell, raised as mindless zombies in the service of the Lich King. Unfortunately, Jaina Proudmoore
, a mage of the Kirin Tor, and Arthas Menethil
, son of King Terenas Menethil
, discovered the plague. Immortality has many faces -- and Arthas quickly killed Kel'Thuzad. But the Cult of the Damned had established a firm foothold in Lordaeron by that time, and as Arthas struggled to fight it, he too fell under the thrall of the Lich King, eventually taking up the sword Frostmourne and becoming a willing servant to the Scourge.
His first task, of course, was to raise Kel'Thuzad from the dead -- and he succeeded by using the Sunwell to power the spell that revived the necromancer as an undead lich, thereby delivering the immortality that was promised. Immortality of sorts, anyway. While everyone knows what happened to Arthas next -- his rebellion against the Burning Legion, the ultimate merging with the Lich King, some people are still unfamiliar with the continued workings of the Cult of the Damned. Now fully under the Lich King's control and no longer tied to the Burning Legion, the Cult continued to spread the plague via the plague cauldrons scattered across the Eastern and Western Plaguelands.
The Cult's purpose is the same as it was when it was originally founded -- to bring Azeroth under the complete and utter control of the Lich King. One of the arms of the Scourge, the Cult trains mortals with a certain bent for power and control, teaching them the dark arts of necromancy and training them in further efforts to spread the plague. To this end, Kel'Thuzad founded Scholomance
, a school for the necrotic arts, in the ruins beneath Caer Darrow
in the Western Plaguelands to train prospective necromancers. Caer Darrow was an old keep, dating back to the days of the empire of Arathor, which had been abandoned during the Second War. The property was bought by a wealthy family, the Barovs
-- who were quick to offer the property to Kel'Thuzad as a suitable spot for his school in exchange for the immortality the Lich King offered.