This quest is no longer available within the game.
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.
NPC #16143 slain
After you bring down the defenses of the Scourge summoning circles, you must 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.