There were a handful of powerful individuals scattered throughout the world who heard the Lich King's mental summons from Northrend. Most notable of them was the archmage of Dalaran, Kel'Thuzad, who was one of senior members of the Kirin Tor, Dalaran's ruling council. He had been considered a maverick for years due to his insistence on studying the forbidden arts of necromancy.
Driven to learn all he could of the magical world and its shadowy wonders, he was frustrated by what he saw as his peers' outmoded and unimaginative precepts. Upon hearing the powerful summons from Northrend, the archmage bent all of his considerable will to communing with the mysterious voice. Convinced that the Kirin Tor was too squeamish to seize the power and knowledge inherent in the dark arts, he resigned himself to learn what he could from the immensely powerful Lich King.
Leaving behind his fortune and prestigious political standing, Kel'Thuzad abandoned the ways of the Kirin Tor and left Dalaran forever. Prodded by the Lich King's persistent voice in his mind, he sold his vast holdings and stored away his fortunes. Traveling alone over many leagues of both land and sea, he finally reached the frozen shores of Northrend.
Intent on reaching Icecrown and offering his services to the Lich King, the archmage passed through the ravaged, war-torn ruins of Azjol-Nerub. Kel'Thuzad saw firsthand the scope and ferocity of Ner'zhul's power. He began to realize that allying himself with the mysterious Lich King might be both wise and potentially fruitful.
After long months of trekking through the harsh arctic wastelands, Kel'Thuzad finally reached the dark glacier of Icecrown. He boldly approached Ner'zhul's dark citadel and was shocked when the undead guardsmen silently let him pass as though he was expected.
Kel'Thuzad descended deep into the cold earth and found his way down to the bottom of the glacier. There, in the endless cavern of ice and shadows, he prostrated himself before the Frozen Throne and offered his soul to the dark lord of the dead.
The Lich King was pleased with his latest conscript. He promised Kel'Thuzad immortality and great power in exchange for his loyalty and obedience. Eager for dark knowledge and power, Kel'Thuzad accepted his first great mission: to go into the world of men and found a new religion that would worship the Lich King as a god.
To help the archmage accomplish his mission, Ner'zhul left Kel'Thuzad's humanity intact. The aged yet still charismatic wizard was charged with using his powers of illusion and persuasion to lull the downtrodden, disenfranchised masses of Lordaeron into a state of trust and belief. Then, once he had their attention, he would offer them a new vision of what society could be - and a new figurehead to call their king.
Kel'Thuzad returned to Lordaeron in disguise, and over the span of three years, he used his fortune and intellect to gather a clandestine brotherhood of like-minded men and women. The brotherhood, which he called the Cult of the Damned, promised its acolytes social equality and eternal life on Azeroth in exchange for their service and obedience to Ner'zhul.
As the months passed, Kel'Thuzad found many eager volunteers for his new cult amongst the tired, overburdened laborers of Lordaeron. It was surprisingly easy for Kel'Thuzad to achieve his goal: namely, to transfer the citizens' faith in the Holy Light into belief in Ner'zhul's dark shadow. As the Cult of the Damned grew in size and influence, Kel'Thuzad made sure to hide its workings from the authorities of Lordaeron.
With Kel'Thuzad's success in Lordaeron, the Lich King made the final preparations for his assault against human civilization. Placing his plague-energies into a number of portable artifacts called plague-cauldrons, Ner'zhul ordered Kel'Thuzad to transport the cauldrons to Lordaeron, where they would be hidden within various cult-controlled villages.
The cauldrons, protected by the loyal cultists, would then act as plague-generators, sending the plague seeping out across the unsuspecting farmlands and cities of northern Lordaeron.
The Lich King's plan worked perfectly. Many of Lordaeron's northern villages were contaminated almost immediately. Just as in Northrend, the citizens who contracted the plague died and arose as the Lich King's willing slaves.
The cultists under Kel'Thuzad were eager to die and be raised again in their dark lord's service. They exulted in the prospect of immortality through undeath. As the plague spread, more and more feral zombies arose in the northlands. Kel'Thuzad looked upon the Lich King's growing army and named it the Scourge, for soon it would march upon the gates of Lordaeron and scour humanity from the face of the world.