Go to a summoning circle and kill a Shadow of Doom, then return to Commander Thomas Helleran at Light's Hope Chapel in Eastern Plaguelands.
After you bring down the defenses of the Scourge summoning circles that have appeared in the zones under attack, you will be able to confront the acolytes who protect them.
In truth, these are not men. They are shadows, some of the Lich King's most frightening creatures. They can be revealed using the necrotic runes they carry, and then destroyed with magic or strength of arms.