Though he had defeated all of the people he now saw as his enemies, Arthas was still haunted by the ghost of Kel'Thuzad. The ghost told Arthas that he needed to be revived for the next phase of the Lich King's plan. To revive him, Arthas needed to bring Kel'Thuzad's remains to the mystical Sunwell, hidden within the high elves' eternal kingdom of Quel'Thalas.
Arthas and his Scourge invaded Quel'Thalas and laid siege to the elves' crumbling defenses. Sylvanas Windrunner, the Ranger-General of Silvermoon, put up a valiant fight, but Arthas eventually eradicated the high elf army and battled through to the Sunwell. In a cruel gesture of his dominance, he even raised Sylvanas' defeated body as a banshee, cursed to endless undeath in the service of Quel'Thalas' conqueror.
Ultimately, Arthas submerged Kel'Thuzad's remains within the holy waters of the Sunwell. Although the potent waters of Eternity were fouled by this act, Kel'Thuzad was reborn as a sorcerous lich. Resurrected as a far more powerful being, Kel'Thuzad explained the next phase of the Lich King's plan.
By the time Arthas and his army of the dead turned southward, not one living elf remained in Quel'Thalas. The glorious homeland of the high elves, which had stood for more than nine thousand years, was no more.