The fight with Lord Marrowgar seems incredibly straightforward, since he really only has four or five abilities. Basically, two tanks start the fight. He shoots lines of fire, which your raid should not stand in. (You've got the "don't stand in fire" trick down at this point, right?)
Periodically, Lord Marrowgar will impale a member of your raid on a bone spike. Kill that spike to free your raid member before that member dies. Lastly, Marrowgar does a whirlwind. That's where the counter-intuitive piece comes in, since you really don't need to run around like a herd of panicked cats to avoid the damage.
I mean, not taking the damage is great if you can avoid it, but the whirlwind doesn't hurt so bad that you should range your healers. But, ultimately, the whirlwind damage isn't all that deadly if you've got a potion, or if your healers are on their game. I've seen more raids die trying to get back together after the whirlwind than raids that die during the whirlwind itself.
Okay, all that being said, let's take a look at Marrowgar's actual abilities.
Saber Lash: Saber Lash is one of Marrowgar's more iconic abilities. It is a massive hit that is split evenly between the tank (technically, Marrowgar's target) and the target's two closest allies. This is, therefore, also the ability that requires you to have two or three tanks. (Two in 10-man, three in 25-man.) If you don't have someone else to help soak the damage, this ability will make pretty quick work of your tank.
Coldflame: Coldflame will target a random member of your raid, and then draw a blue, burning line of fire on the ground between Marrowgar and that raid member. Your raid should simply sidestep out of the flame.
Bone Spike Graveyard: This hurls a massive bone at someone. Whoever gets in the way will get impaled. It really sounds like you can avoid it, but I've never seen a spike happen without someone getting impaled. As soon as a raid member is impaled on the spike, though, all possible DPS should be dedicated to freeing them. If you leave your raid buddy impaled on the spike, they will die from the periodic damage.
Bonestorm: The ability known as Bonestorm actually looks like a big spinning whirlwind melee attack. Most raiders -- especially melee raiders -- are very well trained to not stand in whirlwinds. They instinctively pull away from the whirlwind, so as not to get simply turned into jelly as soon as the ability happens.
This whirlwind, however, doesn't actually do a vast amount of melee damage. If you can somehow manage to avoid it without completely leaving your raid miles away behind you, go for it. Otherwise, just try and sidestep the worst of the hits. Marrowgar will Bonestorm in four different corners, and then return back to his tank. There is an aggro-wipe here, so it's important your tank is ready to pick him up again.
What makes Bonestorm really dangerous isn't the melee attacks -- it's the additional Coldflame lines of fire he puts on the ground when the attack occurs. Just like Coldflame itself, you should avoid standing in it. It's just as deadly as the regular ability.
So, ultimately, if you can manage those four different abilities, you can handle Marrowgar. It's a gear check -- mostly for your tanks, admittedly -- to be sure you're ready for the rest of the Icecrown Citadel.
Boned is the only boss-specific achievement for Lord Marrowgar. You achieve Boned by not leaving any raid members on their bone spike for longer than 8 seconds. That's not a very tough achievement, but it's made easier with the use of a handy "/target bone" macro. Pulling off Boned will probably rely on your ranged DPS characters more than your melee ones, since it'd take a few second for them to get into position.
So, the final summary of fighting Lord Marrowgar is pretty simple. Don't stand in the fire. When he whirlwinds, avoid it if you can, but don't lose your head trying to dodge it. When Bone Spike Graveyard happens, free your raid members as quickly as possible. Make sure you have two tanks soaking the Saber Lash.
The most difficult part of this fight, again, is fighting the urge to completely scatter while trying to avoid the Bonestorm.
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