Here's the basics: Valithria Dreamwalker is a part of the green dragonflight who must be freed. Now, for the specifics...
This is the encounter healers have been waiting for. In order to beat this encounter, you need to heal Dreamwalker from 50% to 100% health. On 10 man, Dreamwalker starts with approximately 5 million health and it's up to the raid to double that amount up to 10 million before it's over. As your raid group is busy working on healing her up, there's more to worry about. There will be a constant stream of trash adds coming into the room attempting to prevent you from healing her up to full. Now you might be thinking to yourself that 5 million health seems like a lot to heal for and it is. There's a special mechanic in the fight that will help you bridge that gap. I'll explain more about it in a bit.
Once you get to Dreamwalker's room, you'll immediately notice that she's being suppressed by these 4 liches. When your group is ready, it's time to open up on them and take them out. It's really easy and they'll go down within seconds. You experience a few Frost Bolts to the face but all you pro healers out there should have that covered, right?
You may wish to plan for a composition that includes Fire Resistance, Nature Resistance, and Frost Resistance due to the nature of the abilities.
These are the trash mobs that you'll be contending with as they stream in. Note that there is a relatively soft enrage timer as it was noticed the spawn time of adds gradually decreased. In other words, they'll get faster as the encounter progresses.
When working on the trash phase, I'd suggest splitting the raid into 2 groups with 1 side handling mobs from the west and 1 side handling mobs from the east.
Let's start with our favorite mobs, the Abominations. These big guys will waddle into the room and infect tanks with Gut Spray. I haven't seen other players get hit with it, so I'm assuming it's just tanks. Try to remove that debuff as it deals 3000 nature damage per second and increases the amount of physical damage taken. It's a manageable buff but don't take unnecessary risks.
Once they're down, a whole slew of Rot Worms will emerge from their corpse. Healers, this is very important. Stay well away from the Abominations when they die. I've managed to pull healing aggro on them a few times. Communicate with the tanks and tell whichever tank is tanking the Abomination to stay and pickup the Rot Worms when they appear. It seemed as if there were around 8-10 worms that would stream out. The Rot Worms do not spawn at the same time.
You'll be able to see these skeletons a few seconds before they enter the area. I'd insist on making them a priority target. The advance notice will help as you'll need your DPS to close within range. Once your tank grabs one, open up on them as much as you can. Lay Waste has the ability to destroy raids if left unchecked. There's no way to interrupt them from what we could tell. They'll also toss Fireballs on players.
These guys hurt. They'll cast a Frostbolt Volley which will hit anyone around for around 4000 and slow down movement speed. The slowing effect is Dispellable. I actually went Discipline just to get a quick Mass Dispel time because I was so annoyed at being slowed down.
Suppressers will run into the room in packs of 3 and go straight for Dreamwalker. They'll cast Suppression which reduces the amount of healing received by Dreamwalker by 10%. They have relatively low health and die fairly quick when AoE'd. You can probably assign a mage or a warlock to deal with them (or whatever AoE you happen to have handy).
Not too worried about these guys. They have a Corrosion ability which has a DoT like attack and an armor debuff component. If they decide to Acid Burst, then they'll explode with the raid taking a high amount of damage followed by an additional 1000 per second after that.
So what's the order of operations? I'd suggest starting off with Blazing Skeletons, Archmages, Suppressers and Abominations or Zombies. You can probably let Suppressers come in for a while. Just don't bother healing Dreamwalker until Suppresser's get taken out by the raid all at once. Blazing Skeletons are the most immediate threat, however.
There are two things to watch for on the ground.
Column of Frost will appear as a blue circle on the ground before erupting in frost and sending any players standing on top of it flying into the air. Don't stand on fires.
Mana Voids appear as void zones on the ground. Players standing on top of them will experience a mana drain at a rate of 1000 per second. Having a large mana pool will help just in case you're a bit slow getting out. If you find yourself with low mana because you weren't paying attention, duck into the portals that I'm about to mention.
There's just something about portals that green dragons are attracted to and Dreamwalker is no different. Periodically throughout the fight, green portals will spawn that your raid members can jump into. They'll teleport you into a phased part of the fight. You'll notice green spheres floating about you. If you kill them within 10 yards, you'll gain a buff to your damage, healing and mana regeneration called Emerald Vigor. The green spheres only have 100 health. Feel free to fly into them (yes, you can fly just like in the Kael'thas encounters) and spam Holy Nova or Arcane Explosion.
More importantly, the Emerald Vigor buffs stack. Each application is worth 10% extra damage and healing as well as 200 mana every 3 seconds. As you can imagine, these will play a key role in the fight. I've managed to assemble a stack of about 12 at one time (that's 120% to healing). After about 15 seconds inside, you'll be phased back into the encounter with the buff still active (Emerald Vigor lasts 30 seconds) so make use of it healing the raid or pounding extra heals on Dreamwalker.
Here's the catch 22 that we've run into. Sending a healer and 2 DPS inside the portals means less DPS on the outside for handling trash (remember, the speed at which trash spawns gets gradually faster). Send 2 healers and 1 DPS might mean your outside healer may buckle under the pressure. In the end, we opted to send only 1 healer and 1 DPS inside (a holy paladin and a mage). We felt that a paladin could beacon Dreamwalker and heal the tanks or whoever needed healing at the same time. Their single target healing done would be amazing. The mage would help with overall AoE on Suppressers and could deliver crushing DPS on priority targets.
Emerald Vigor seems to last just long enough for it to wear off before your team can jump in again. You can't stack it to obscenely high numbers unless you can catch green orb just before you phase out. It's very tricky and unlikely.
Keep track of how long is left inside the dream phase before you phase back into reality. You don't want to fly too high and plummet to the ground when you get back in.
And that's the extent of the fight really. Every chance you get to renew a HoT on Dreamwalker, do so. Any chance you can squeeze a healing spell on Dreamwalker, do so. You may wish to establish cycling teams of healers going into the portal phase to rotate buffs and regenerate mana simultaneously. I don't advise sending in more than 1 at a time as you might get overrun on the outside. There's no way to get out of dream phase other than waiting for it to wear off.
There we have it guys! Here is genuinely unique encounter where we have to heal an ally to full. I'd say the encounter needs some tweaking of the numbers themselves. The Frost Volleys do get annoying after a while. I'd rank it a 7/10 on my personal scale for healing enjoyability.
I can't wait for pickup raids to get to this point. I can envision trade chat filled with scores of something like, "LFM healers, 7000+ HPS" which may or may not be conceivable.
For once, healing meters just might mean something when taken literally.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to the Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.