I have a certain perspective on encounters. I operate both as a tank, and as a raid leader. Most guides that I have seen in the blogosphere have been written from the perspective of a healer. Not sure why, perhaps the same personality that's inclined to write a guide is inclined to play a healer. But as I read these guides from the perspective of both a tank and a raid leader, there's a lot of information that I wind up having to sort out on my own, because my perspective, and the perspective of the people who write the guides often do not concern all of the same issues. At the same time, I read a lot of post from bloggers who are looking for that same information that I had to figure out on my own. So, I guess it's time that I share some of that information.
Lord Marrowgar is the first encounter in Icecrown Citadel. It is a two phase encounter.
Phase One: Begins at the pull. Marrowgar will be picked up by the tanks, while the DPS attack. Twice per phase 1, Marrowgar will cast Bone Spike Graveyard, impaling 3 players on 25 man, one on ten man. These bone spikes have health, and must be DPSed down to free the impaled player before he dies. He also launches out a path of Coldflame which is aimed at a raid member farthest from melee range. This may occasionally be the tanks if your raid stacks within Marrowgar's obese hit box. After two Bone Spike Graveyards, he will transition to phase two.
Phase Two: Marrowgar will initiate a whirlwind, and spin about the room like an arms warrior on crack. He will move to a random spot in the room, launch Coldfire in four directions, and then spin to another location. He will repeat that process several times before dropping aggro and transitioning back to phase one.
Tanks: You will need three tanks on 25, two on ten. The major mechanic that governs the tanks is Saber Lash. All this requires is that the tanks stack together to split the damage. Imagine it as a cleave on steroids.
Establishing dominance: At the beginning of each repetition of phase one, there is a full aggro drop. Marrowgar is now tauntable, however, it is still advised to move towards his final whirlwind destination, and the damage from the whirlwind is somewhat negligible, and it is much more advantageous to have some additional time to build threat before some overzealous DPSer decides to get himself killed.
Gearing philosophy: Given that the only non physical damage is the Coldflame, which you shouldn't be standing in, and saber lashes are avoidable, and mitigatable, pretty much everything works to one degree or another. Armor, Stam, Avoidance, it's all great for this fight.
Tank Death Scenario: The risk of tank death in this fight primarily occurs at the transition between phase two and one. As the tanks pick aggro back up, if they are not stacked correctly, this may lead to the primary tank taking an additional 50%-200% damage from a saber lash being not properly shared. He doesn't hit that hard in easy mode, but you'd still be talking about a hit in the 40k range on 25 man on a well geared tank.
The Raid: Positioning in phase one is not extremely important, so long as no one is moronic enough to stand next to the tanks. Generally you want to keep people fairly close together in phase one, so that bone spikes can be downed quickly. In phase two, it's pretty much just run for your life, and don't stand in the fire.
Outrun or Outlast?: Definitely outlast. The fight can be controlled. The enrage timer is absurdly generous. If your tanks have got glass jaws, and you need to bring 9 healers to keep them alive, you can get away with that. At the same time, the majority of the non tank damage is avoidable, so as long as you've got decently geared tanks, you could get away with as few as four healers.
Wipe Scenario: It's a slow death in this encounter. It'll typically begin with a healer getting either saber lashed during the transition back to phase one, or bone spike in the coldflame right before the start of phase two. The tank healing becomes strained, and eventually you lose a tank. When one tank goes down, it greatly increases the incoming damage on the other tanks, and they wind up dropping. The key to avoiding this is quick downing of bone spikes, and personal awareness of positioning.
Shift Fire: The only meaningful DPS shift in this fight is jumping from the boss to the bone spikes, and back onto the boss. In 25 man, to optimize speed, you may wish to use a program like DBM, that will place raid marks on the spikes, and assign a group to free each mark.
Heroism: Heroism is best used as soon as the tanks have enough aggro. It promotes synergy with offensive cooldowns, and the initial phase one provides the longest time on target. There are no frenzies or soft enrages to deal with.
The Fire: In this fight, the fire is blue moving fire on the ground. The fire is also a giant screaming whirlwinding skeleton. Don't stand in the fire.