This quest is not available in game.
Defeat Lady Deathwhisper while ensuring that Darnavan survives.
We had hoped to place a man on the inside of the Cult of the Damned, to give us an inside angle on the Lich King's machinations, but we... underestimated the persuasive powers of Arthas and his dark minister, Lady Deathwhisper.
Our man, Darnavan, is one of them now.
If you plan on fighting your way through the Oratory ahead, please keep your eyes peeled for Darnavan. If you can bring him back to us alive, we can attempt to reverse the Cult's brainwashing.