Found in the Oratory of the Damned, Lady Deathwhisper is the supreme overseer of the Cult of the Damned, a fanatical group loyal to the Lich King.    

After the raid group had defeated Lord Marrowgar, they ascend a small ramp to the entrance of the Oratory of the DamnedAt this point, a small teleport pad will become active, allowing the group to teleport from the front entrance of the Citadel to the Oratory.

Lady Deathwhisper - Phase One
When the fight begins, Lady Deathwhisper protects herself with a shield that converts all incoming damage to a reduction in manaOn 10 man, she has approximately 3,000,000 mana to burn through, and the same in hit points once the shield is downShe has an enrage time of 10 minutes.

During this phase, she does not move from her position at the front of the room, but instead summons adds the party must kill and also casts spellsShe will continually cast uninterruptible Shadow Bolts at random party members, and also drop a Death and Decay at various locations around the roomThe mechanic for this Death and Decay is the same as a Death Knight's, but the icon on the ground will be green instead of red, so players will know what to look for and not to stand in it.

In 25 player mode, Deathwhisper will also cast "Dominate Mind" on a player, mind-controlling them and increasing their damage done by 200% and their healing taken by 500%These affected players must be crowd controlled to prevent them from wiping the raid.