A shadow crossed Rex's face. The undead themselves were not frightening. What frightened him was collusion between the undead and humans. That was something almost impossible to guard against.
Rex knew little about Pua Stormbringer, the mortal leader of the Cult of the Damned. He had only a rough idea of her background. Early in the Third War, she had answered the Lich King's call, joined the Cult of the Damned, and begun serving it.
The fact that Pua Stormbringer could hear the Lich King's call proved that she was no weakling. Not everyone could hear his summons. Those who could invariably harbored darkness in their hearts, possessed powerful obsessions, and had considerable strength.
That was also why Rex had not immediately destroyed the Cult of the Damned after Kel'Thuzad founded it. Every powerful individual who heard the Lich King's call was a hidden threat. Their strength was secondary; what mattered was their standing. Many held prominent positions. Take the plague spreading through northern Lordaeron, for example. At first, apart from the Plague Cauldrons, it had spread through contaminated grain. The Cult of the Damned could easily infect the grain under its control, then sell it, spreading the plague throughout northern Lordaeron.
Vincent's idea was sound: use the Holy Light to identify any enemies who might have infiltrated their ranks. But the Holy Light was not omnipotent. Against living people who yearned for darkness, willingly colluded with the undead, and obeyed the Lich King's commands, it was of little use.
After passing through the gates of the Greymane Wall, Rex summoned Osborne and ordered him to begin watching Pua Stormbringer. Rex did not act immediately because he could not estimate how many members of the Cult of the Damned had slipped in among the refugees from Lordaeron. Screening them with