With Marrowgar down, next up on any raider's dance card is the somewhat chatty boss, Lady Deathwhisper! So here is #2 in what I hope to make a series on ICC bosses, featuring tips for Disc priests.
Deathwhisper is a two-phase boss, of the front-loaded variety (This means phase 2 is easy, so that it may still be completed if your DPS die to completely avoidable damage, and/or your healers have a coronary trying to heal DPS who are dying to completely avoidable damage).
Getting past phase 1 relies upon the tanks being able to pick up the adds, while the DPS burn down adds in the right order. Unfortunately you can't help with that, but you can make sure your assigned tank(s) stay vertical. I recommend keeping your tank in sight: make sure you stay in range and in LoS (note the stairs in the room). If your guild brings all the adds together by the boss, this is easier, but there are still steps and pillars to watch out for.
Also try to be aware if a Deformed Fanatic is activated. If one is, it may be worth it to use Pain Suppression, or at least a Shield-Penance-GHeal cycle or two. Deformed Fanatics are usually kited but if they get too close, your shields could save the day.
As for the DPS, you can help them too. Configure your UI to display people standing in the Death and Decay, and those who have agro. Shielding both can add a nice safety margin to DPS who are already watching add spawns. You can further help by dropping a few DoTs on Deathwhisper in between trash waves if you like.
Finally, watch out for mind controlled players, as they may get healer agro. If targetted, use Fade instead of Psychic Scream: the radius on the fear may also send adds fleeing, and this tends to make tanks cranky. Alternatively you can DPS down the mind controlled players; this tends to result in hilarity, but not generally success.
Once phase 2 hits, it's more of the same: heal tanks, don't stand in the fire. However, be sure to watch where you're standing. Vengeful Spirits will spawn from the dead adds, and if they reach you they can hit for 20k!
POWER INFUSION TIME
The time to cast PI is early and often. Cast it on the players assigned to burn down the boss's shield: since they won't switch targets, they'll get the most use out of it. If your guild burns Heroism early, cast it right after, otherwise cast it on the top boss-killer DPS on cooldown.
BONUS STRATEGY HAIKU
Heal tanks, shield raiders
And if you get mind controlled
Try to kill a mage
Deathwhisper is not a hard fight to heal... most of the learning curve is dealing with the adds. But some good ol' Disc-elbow-grease can make it easier on everyone to handle!