Lady Deathwhisper is the second encounter in the Lower Spire of Icecrown Citadel.
Phase One: (10 man) Lady Deathwhisper will remain stationary, casting shadowbolts at random raid members, and throwing out green death and decays. She will spawn adds. These come three at a time. The first spawn will be on the left hand side, and have two Fanatics (melee), and one Adherant (Caster) in the middle. The next spawn will be on the righ side, and consist of two Adherants, with a fanatic in the middle. Spawns will alternate sides. She will occasionally reanimate, empower, or transform these adds. You will need to tank and kill these adds, while DPSing through her mana shield to enter phase two.
(25 Man) The gist of the fight is similar. The major differences is that there are now seven adds spawning, the three on the left, the three on the right, plus one random add in the back. She will also occasionally mind control a player, requiring you to CC them.
Phase Two: Phase two begins with a complete aggro drop, she ceases spawning adds, becomes mobile, and begins using a melee attack and a powerful frostbolt on her main aggro target, in addition to her abilities from the previous phase. She will no longer MC players. She begins applying a debuff on her main aggro target called Touch of Insignificance, which reduces their threat generation by 20%, and stacks up to five times. This will neccesitate tank swaps, unless your DPS can do magical things with their threat generation. The frostbolt should be interrupted, lest it do a large quantity of irressistable frost damage to the tank. A rotation will be needed in 25 man, however, any competent warrior, rogue, shaman, or DK can do it in ten man. She will also spawn ghosts, that pick a target, and explode after a time interval. they need to be kited away from the raid during that interval.
Look, She Brought Friends!: There are many adds in this fight.
Fanatics: These are melee mobs. They hit hard enough to one shot most clothies, and feature a cleave, so face them away. If they become reanimated, they gain a shield that makes them immune to physical damage, thus ranged DPS need to burn them down. If they become Deformed, they start hitting like a freight train. 35K+ on a well geared tank. However, they move slowly, and can be strafe kited with ease.
Adherants: These are caster mobs. They cast shadowbolts which hit for about 10k. If need be, they can be tanked by a melee DPS. If they become reanimated, they put up a shield that reflects all spells. It does not however, reflect weapons, so have your melee DPS lodge theirs in it's skull. If the become empowered, they immediately begin spamming AoE spells, and inflict significant damage. Empowered Adherants should always be a priority.
Tanks: Two tanks will be required to allow for the switches in phase two. One tank will be needed to tank each side in 25 man. A third tank can be helpful to pick up the add in the back, but is not needed.
Establishing Dominance: The pickups in phase one can be a little sticky. The adds spawn far enough away from each other that cleaves and other multi mob pickups are ineffective. DPS need to be disciplined, and wait for the tank to complete the pickups. A hunter for misdirects is a godsend. The mobs drop aggro if they become deformed, empowered, or reanimated, and must be reaqquired ASAP. When you enter phase two, you need to have a tank ready to taunt as soon as the barrier drops, otherwise she'll start attacking the DPSers who weren't smart enough to pull out. When you reach the tank transition, if you are the tank taking her, it's important to build as much threat as possible during the first stack, so you aren't gimped when you get two or more stacks. If you're the tank switching off, you don't need to worry about threat management as much as you would on say, Festergut, because the debuff will ensure you don't take it back.
Gearing Philosophy: The actually difficult part of the fight in terms of tank damage is usually phase two, and most of that is magic damage. Feel free to go straight stam for this fight, as armor and avoidance won't serve you too well.
Tank Death Scenario: There's two potential points of tank death. A careless tank who does not pay attention to the adds once secured, might accidently find himself tanking, rather than kiting, a deformed fanatic. 35k melee hits plus other incidental damage can be painful. Keep in mind that if a Fury Warrior or Feral Druid is MCed, the tanks will be crittable, and in this case, instagibbed. Always kite the deformed fanatic immediately. The second scenario is an uninterrupted frostbolt, typically combined with a melee hit and splash damage from a ghost or D&D. This is preventable by having a skilled interrupt cycle set up.
The Raid: The raid should have prexisting assignments for which adds they DPS, and who CCs the MC target on 25 man. In ten man, a coordinated quasi zerg is sufficient. All DPS on adds, then all DPS on boss, then back on adds, then back on the boss. However, in 25 man, the increased number of adds, and increased mana pool typically sees this strat ending in failure. I've found it better to assign a group to each side, and then setting up the remaining DPSers as full time on the boss. The number of DPSers you leave on deathwhisper depends on how well your raid can handle the adds. We began with 2 on our first kill, but now we leave seven on her to push out of phase one ASAP. The fight is easily healed with five healers, so long as your DPS doesn't camp in the fire.
Outrun or Outlast?: Outlast. While the enrage isn't as relaxed as Marrowgar's, you've got plenty of time. The important thing in this fight is to make sure that adds are dealt with properly.
Wipe Scenario: Most wipes on this encounter occur in phase one. An add isn't picked up, or is pulled off carelessly, and winds up eating a healer or two. If you fall behind on the adds in particular, the pickup becomes very difficult. Makes sure your tanks are tanking their assignments, and make the the DPS are watching omen. Also, ensure you have enough DPS assigned to adds that they all die before the next wave spawns.
Shift Fire: This fight will involve a lot a changing targets in phase one. Every add wave needs to be dealt with. Any spare time between waves should be spent burning the mana shield down. As she mutates adds, it's important to shift your DPS accordingly. Kill order should be Deformed>Empowered>Reanimated Caster>Reanimated Melee>Melee>Caster.
Heroism: There's two optimal points for Heroism, either right before, or right after the phase two transition. Use it before phase two if you have trouble hitting phase two before the next add wave spawns, this gives you a lot of breathing room on the enrage timer. If you're clean about the adds, then it would be optimal to save it for after phase two starts, so that the bulk of it is spent with the most DPS possible on the boss.
The Fire: In this fight, the fire is green bubbly runes and blueish green ghosts. It may also be any AoE that your MCed players can use. Don't stand in the fire.