Undead scourge

From Ujimasa's map comendium:

Even before the Horde was defeated and the Dark Portal was destroyed, the Burning Legion had already enacted another plan. Kil'jaeden sent his Dread Lord agents to find another way to cripple the forces of Azeroth and prepare for their invasion. The Dread Lords would infiltrate the upper echelons of the different kingdoms hoping to exploit their weakness when the time comes.

Among them was Tichondrius the Darkener, who dared infiltrate the magical Kingdom of Dalaran. He became interested in one of the members of the Council, Kel'Thuzad who secretly studied necromancy. Tichondrius eventually revealed himself to the archmage promising to teach him more about the forbidden arts. Enthusiastic at the prospect of delving into necromancy, Kel'Thuzad accepted Tichondrius' offer. They relocated to the colonies in Northrend in order to avoid the prying eyes of the Kirin Tor.

During their stay in Northrend, news regarding New Arathor's transgression back in the Eastern Kingdoms reached them. Seeing this as the perfect opportunity to strike, Tichondrius offered Kel'Thuzad the Helm of Domination augmenting his new-found abilities even further. They then preyed on the isolated Human settlements, slaughtering them before raising them into the Undead. With their Undead armies, they would also bring the Nerubian Empire into their fold. With their power base in the north solidified, Tichondrius sent Kel'Thuzad back to the Eastern Kingdoms in order to undermine its defenses to the newly established Undead Scourge.

Back in Lordaeron, the Barovs of Scholomance offered Kel'Thuzad their home as a place to train new apprentices in exchange for protection against New Arathor's armies. They were unaware however of the horrors that Kel'Thuzad plan to unleash.