Lord Marrowgar is the first encounter in Ice Crown Citadel and is fairly easy to down.
This boss tests if the raid is able to survive Icecrown Citadel. If the raid wipes then they will probably not survive through the other bosses. His main attacks are Bone Storm and Bone Spike Graveyard. He has an enrage timer (10 min), so it is not exactly a DPS race.
Bone Storm - Attacks nearby enemies in a whirlwind of bone that lasts until canceled. 3 sec cast. Every two seconds, triggers AoE damage that drops off as you get farther away from the boss. Roughly 6k standing on top of him, 4k just inside his hitbox, 2k just outside, and 800 or so at around 30 yards. The boss charges 4 targets before ending the Bone Storm. 30 sec cooldown.
Bone Spike Graveyard - Hurls a massive bone spike which impales any enemies in the way. Deals 10% health per sec for 5 minutes or until killed. 3 sec cast. 39k hp. 10 sec cooldown.
The DPS must break the person out of the graveyard
before it kills them.
Coldflame -Summons a line of frost that hits enemies in the way. Instant. 8 yd line of effect. Deals 6000 Frost/sec for 3 seconds. Random target.
Bone Slice - Melee range—Splits 200% of normal melee damage to an enemy and its nearest ally, replacing melee attack. Used every second, but not during bone spike graveyard cast; prevents the targets from being targeted by Bone Spike Graveyard. If there is no one near Lord Marrowgar's target, that target takes the full damage of Bone Slice.
After 10 minutes, Marrowgar will Enrage, increasing his damage done by 500% and attack speed by 150% and making him immune to taunt.
Tanks need to stand on top of each other to soak Bone Slice. If you are short on tanks and the raid has plate DPS available, one can be put in front of the boss with the tanks to help soak the Bone Slice damage. If said DPSer has a tanking set, equipping a piece or two to increase their HP is an option should they be taking too much damage. Note that the tanks will take substantial damage during the fight - using an off tank of this sort is not advised, as it will make healing even more difficult, although it is better than insufficient tanks.
It is entirely possible to single tank Marrowgar, in 10man, should the tank be sufficiently geared. In 3.3(a) Marrowgar's damage was further reduced, making this an even more viable strategy. Be advised that although this frees up another DPS in your 10man raid, the tank will take heavy damage.
The key to the fight is managing the transitions between the Bone Storms and going back to the Bone Graveyard phase. You have 10 seconds from the end of Bone Storm until Lord Marrowgar starts casting Bone Slice again (until then he simply melees his target) - the tanks need to pick him up and position themselves together in that time. (The 10 second grace period also applies to the beginning of the encounter, counting from when he is first aggroed, giving the tanks time to set themselves up.)
Tanks should avoid sandwiching themselves between Marrowgar and a wall. The trails of fire need a place to land behind the tanks, or they will be cast inside the hit box, affecting the tanks.
Lord Marrowgar has a gigantic hit box You will want to have a dedicated 'spike team' - chosen dps members who will kill the Bone Spike Graveyard as soon as it impales a player. If you're doing 10-man without a dedicated spike team, the melee need to be grouped near the healers until you out-gear the encounter, or are attempting his heroic mode. Melee can choose to stay close to the boss to avoid the Coldflame, which starts stay out of range of the 8 yd Coldflame and still attack the boss.
Ranged DPS, or nearby melee, should immediately free players impaled with Bone Spike Graveyard. If the raid has an abundance of melee, members should stay relatively close together to reduce travel time. Bone Spikes should die within 3 seconds (two GCDs); any longer and you will have issues. In addition, it is confirmed that Paladin ability Hand of Reckoning removes Bone Spike Graveyard, and it's most likely that similar abilities remove it too.
Coldflame is a single line of frost that runs radially outward for 8 yards. Touching it inflicts a large amount of damage. Since it moves in a straight line, melee can sidestep out of its way, while ranged players can avoid it entirely. Because it starts a small distance away from Lord Marrowgar, melee can also avoid it by staying in very close, but only do this when Lord Marrowgar is not casting Bone Storm. In heroic, touching the line of frost will apply a bleed effect.
Cold Fire during his non-Bone Storm periods will be directed at a given person, thus ranged should spread out radially, preventing more than one from having to move from any given spawn.
very 30 seconds Marrowgar will execute Bone Storm. This whirlwind attack cannot be avoided, and damage is dealt every two seconds based on your proximity to the boss as he spins. The closer you are, the more damage you take. Additionally, in heroic this will also leave a bleed effect on anyone hit by it.
While in his Bone Storm Lord Marrowgar will randomly target and charge a raid member. When he reaches them he stops moving for around six seconds before targeting another random raid member. Every time Lord Marrowgar reaches his target and stops he will fire four lines of Cold Fire along the semi-cardinal points (NE, NW, SE, SW - they form an X). Unlike during phase 1, the cold flame lines start directly underneath him. After his fourth target the Bone Storm will end.
Tanks will need to be close by to absorb Bone Slice when Bone Storm ends.
The tanks take a fair amount of damage during combat (10-15k damage every second from Bone Slice, or 13-17k damage on 25 man).
During the Bone Storms, every raid members takes damage. People who don't move when they're targeted by the boss during Bone Storm will be taking full Bone Storm damage in addition to Cold Fire damage from up to four separate Cold Fire effects, which will probably result in their death.
Note that Lord Marrowgar has a red positioning reticule below him. Stacking your entire raid (with the exception of the three tanks and any hunters) on the back edge of the reticule does three things:
Prevents all incoming Coldflame damage.
Controls the location of Impales so they may be quickly DPSed.
Allows Healers and Caster DPS to remain stationary, maximizing throughput.
The raid must disperse during Bone Storm. Bone Storm damage is negligible, but Priests need to be mindful that Inner Fire charges are depleted by this ability. Stay mobile, and re-collapse to the rear of the positioning reticule once the tanks regain control after it stops.
Note that tanks do not take damage during the Bone Spike Graveyard cast, which can be very handy for moving into position or catching up on healing.
Tips: During bone storm try only to move to avoid the cold flame, it is easy to heal through the bone storm. Simply keep an eye on Lord Marrowgar to see whether or not you will be in the way of the cold flame
The same strategy you use in normal mode should work in heroic mode, as the fight is largely unchanged though with drastically increased Bone Storm damage. The cold flame lasts longer and if you use a strategy where everyone stands inside Lord Marrowgar hitbox, your tanks will need to work around 3 or more cold flames up. During the bonestorm phase, Lord Marrowgar will continue to cast bone spike. This could lead to easy deaths if the bone spike sits on top of a cold flame, with Marrowgar spinning at the same location. Players caught in this situation need to be vocal and ask for a healer cooldown to survive.
[info from http://www.wowwiki.com/Lord_Marrowgar]
You can also look at this video below if you prefer to see it done: