Magmaw is the first boss in Blackwing Descent, and will be the first raid boss most people tackle in Cataclysm. It's a relatively simple fight revolving around basic add management and moving out of bad things.
One of the most crucial elements in this fight is controlling the Lava Parasites that spawn periodically. With Death and Decay to hold threat on the parasites, and Howling Blast for powerful area of effect (AoE) and slowing capabilities, we are perfectly and uniquely suited for this task. While Howling Blast isn't quite the AoE monster it used to be, it still does a nice job on the worms. Also, if you have no other class capable of AoE slows (such as a Hunter's Ice Trap) you will probably want to spec into Chilblains.
Get your running shoes on, though - you're going to be doing a LOT of back-and-forth travel in this fight!
Your special responsibilities:
• AoEing the worms, kiting (& slowing if specced) as necessary
When the encounter starts, I DPS Magmaw like normal but turn the camera to look back at the ranged group, so that I can run out and join them as soon as I see the lava circle appear. There's no requirement to stack here, but being clumped up helps healers, especially Shamans.
I usually try to get a Rime proc on Magmaw before the worms spawn for a bonus Howling Blast, but it's not always possible. About ten seconds before they show up, I stop using Obliterate because if Rime doesn't proc, I've just wasted a Frost Rune which could have been used on a Howling Blast. Dealing with the worms is MUCH more important than a little extra DPS on Magmaw, so this is bad!
I drop a Death and Decay on the lava circle for initial aggro (usually while running to the ranged group), then Howling Blast the worms as much as possible as they pop up. Try to overlap your Death and Decay on the lava circle - if you can get fast initial threat on all the parasites, any raid members who got knocked up into the air won't get their faces nommed when they land.
Really important note: DO NOT LET THEM REACH YOU, EVER. Keep far away, even if you have to run away and just sit there while your runes recharge. If the worms reach you they hit you with Parasitic Infection (horrifying tooltip, btw), a DoT which after a short time triggers Infectious Vomit, dealing AoE damage to everyone around you and spawning MORE parasites. THIS IS VERY BAD.
Some DKs switch to Blood Presence for the additional threat. Personally I've never found it necessary, but if your raid has some real AoE beasts that will out-threat you on the worms, just pop Blood Presence as you're running out to the ranged group. Try to use up all your Runic Power beforehand though, as switching presences makes your RP revert to zero.
At this point, if Howling Blast will be up again soon (3-4 seconds), I'll wait at range and Death Coil Magmaw a few times, Horn of Winter, etc. If Howling Blast won't be ready for 5+ seconds, I'll run back into melee (on the opposite side of Magmaw as the worms, obviously) and start normal DPS while keeping an eye on the worms.
Since I have aggro on the parasites, they'll slowly make their way towards me, giving the ranged DPS plenty of time to finish them off. If they're not dead by the time Howling Blast is up again, or if they start getting close, I kite them away from the melee and help the ranged DPS finish them off with another Howling Blast or two.
A few warnings: be sure NOT to run out to the ranged before the lava circle shows up! If you're halfway between the melee and ranged when it spawns, it could spawn on you next to the melee group and cause all sorts of problems. Wait until you see it, then head out.
Also, if the chains don't go down fast, you may end up with parasites spawning immediately prior to Magmaw's head being impaled. This sucks, but your job is to handle the adds first and foremost, so aggro them up and kite them away. Forget about DPSing the exposed head until they're all dead - your ranged should be helping you burn them down. If the two people handling chains are fast, you should never encounter this, but nobody's perfect! It happens.
And that's basically it! The rest of the fight is simple. Like everyone else, when half the room fills with steam you want to move to the other side. You may want to retreat to the ranged group when the tank gets eaten - Magmaw tends to wildly crush anyone in melee range during a Mangle period, and I've taken some scary 90k+ hits that have almost one-shot me.
Our raid Bloodlusts the first time Magmaw's head is exposed, so that's a good time to use Army of the Dead. I do it as soon as I see both chains land successfully. While this prevents the ghouls from benefiting from Bloodlust, I feel the lull right before Magmaw crashes down is perfect channeling time.
Magmaw has some nice treasures for Frost DKs, such as Lava Spine for Dual Wielders, and Akirus the Worm-Breaker for two-handers. Akirus is amazing, and Lava Spine, while not perfectly itemized for us, is still a very nice upgrade.
Here's a video demonstrating everything I've just described. We got Parasite Evening on this go, which if you're doing your job correctly, is a pretty easy achievement.
PS: When you run back into melee range to DPS Magmaw, don't right-click him to attack if you have click-to-run enabled! Despite the fact that his hit box is like, THE ENTIRE ROOM, I've once missed and ran right off the edge into the lava. Quite embarrassing. >_>