[Edit: I've been made aware of the fact that my commenting isn't working for some reason. So, to those of you that have just been dying to leave me a fart joke or something, I apologize and I'm trying to get to the bottom of it.]
I would've been able to say that I was writing this while plowed my way through a 10m ICC PuG and written the encounter 'as it happened,' per se, but no, I got saved to a 10m ICC PuG earlier in the day that was already on the second boss when I entered the instance and promptly got disconnected. Yay!
Anyway, so you've just downed Marrowgar, maybe got some loot, and are trodding up the stairs behind him. Eventually you'll reach the top of the stairs and be looking out over the room containing the Lady herself and her few packs of adds.
The trash itself is pretty simple; your tanks will pull one pack at a time, while everyone else should be around one of the corners, LoS-ing the mobs until they reach the top of the stairs. Then just cleave, cleave, cleave away.
After these two packs are done, the only trash left before the boss are the two large guys on either side. Fairly simple tank n' spank, however, if you get a debuff on you (blanking on the name) run away from everyone else.
There are two phases to the Lady Deathwhisper encounter. The first phase is primarily add management, while the second phase is your standard 'kill the boss' phase.
On 10m, your raid should split into at least two groups: a tank and some DPS for the adds and some ranged on the boss. And some healers. Ok, maybe that's three.
On 25m, you should split up similarly, only now each side of the room spawns adds every time, so each side should have its own dedicated add-killing team of tank and DPS. Also, your third tank should be positioned in the back of the room to pick up the errant add that spawns back there as well. Oh, and some more healers.
I should perhaps explain why there are a few ranged DPS on the boss. Deathwhisper has a Mana Barrier, similar to a mage's Mana Shield, that protects her from harm by deducting damage taken from her mana instead of her health.
(Technically, damage she takes is first deducted from her health and then replenished from her mana, like a continual healing effect, but what ever.)
It is only when this barrier is down can you actually kill her. So your ranged DPS that are assigned to the boss will be slowly chipping away at that barrier. Anywho, let's get on to the first phase.
When you "pull" Lady Deathwhisper, she will throw up her barrier and start summoning waves of adds on a timed basis (your DBM should inform you when the next wave of adds are gonna spawn, just an FYI). Deathwhisper spawns two types of adds: Cult Adherents and Cult Fanatics. Adherents are the ones that you want to take down first, as they toss up Shroud of the Occult, preventing ranged from really doing anything to them. Plus they're fragile little clothies, and we ALL know how fun it is to tear a poor clothy apart.
After those are down, you should grind down the Fanatics, and if you're on 25m, assist in taking down the add at the back of the room.
Periodically, an add will cast Dark Martyrdom, which will kill it and allow it to be resurrected as a skeleton. These need to be burned quickly. If it's a Reanimated Adherent, go to town. If it's a Reanimated Fanatic, let your ranged deal with it. However, if you're DPS is high enough, you could completely avoid this.
Anyway, this will continue for a while until your ranged can bring down Deathwhisper's barrier. It's best to time this between add spawns, for obvious reasons.
Phase two begins as soon as her barrier drops, allowing you to actually do damage to her. This is mostly the burn phase, except that you need to watch out for the following:
No, they're not going to help you with ghosts, stop calling them.
- Ghosts. They're spawn periodically and chase people. Right before they die all a'splody-like, they cast Vengeful Blast, which... hurts. Just try to get away from them.
- Death and Decay. Deathwhisper will spawn one of these every once in a while, which appears as a bright green circle on the floor. MOVE. That's it.
- Frostbolt. These should be interrupted because... well, simply because they can be interrupted. Realistically, we are not the go-to class for interrupts, but if you can toss an extra interrupt in there every once in a while, especially if someone says they can't get the next interrupt, go ahead.
- Dominate Mind. The Lady only uses this in 25m, so don't fret about it in 10m. Works just like any other Mind Control mechanic, just CC the affected individual for 12 seconds and you should be good.
On 10m, some loot you might want:
Whispering Fanged Skull Just an amazing trinket, apparently has a 35% proc rate and a 45 second internal cooldown, meaning that it will be up often.
Deathspeaker Zealot's Helm Pretty darn good helm, just remember not to break your 2pT9 until you get 4pT10!!
On 25m, some loot you might want:
Blood-Soaked Saronite Stompers BiS boots before heroic ICC. GET IT NOW!
Ahn'kahar Onyx Neckguard Also BiS neck before heroic ICC. ROLL ROLL ROLL!
Stay tuned for the Gunship encounter!