World of Warcraft: Shadow of the Necropolis

In the Lich King's haste to spread the plague of undeath over Azeroth, he gifted one of his greatest servants, the lich Kel'Thuzad, with the flying citadel of Naxxramas, a horrific base of operations for the Scourge. Consistent attacks from the Scarlet Crusade and Argent Dawn factions weakened the defenses of the floating fortress, enabling an incursion that led to Kel'Thuzad's defeat. However, a traitor among the ranks of the knightly order of the Argent Dawn absconded with Kel'Thuzad's cursed remains and fled to Northrend, where the fallen lich could be reanimated.