Even if he's not perfectly tuned yet, the testing has given us some idea of how his fight will go. His notable abilities are...
- Whirlwind - Just what you would expect, a physical damage AOE. When it lands, it also applies a DoT. Lord Marrowgar will announce the ability before he uses it, so your melee will want to get out of the way so they don't get hit with the bleed.
- Saber Lash - Seasoned raiders will know this ability well. It's an incredibly powerful melee strike that splits its damage across three targets (on 25-man, two targets on 10-man) so you'll want all of your tanks standing together. That way, nobody gets 1-shot by it.
- Coldflame - A line of (cold) fire spawns, and you must not stand in it. Do not stand in the fire. Seriously, people.
- Bone Spike Graveyard - Lord Marrowgar throws a bone spike in a chosen direction, and anybody directly in the line of fire will be impaled. The rest of the raid will need to DPS down the spike to free impaled players. It can be avoided, and it will not hit targets being hit with Saber Lash.
The fight (which is basically a Black Temple flashback) is mostly keep the tanks alive, move out of the Whirlwind, don't get impaled, and then loot the boss. Considering there will need to be two or three tanks in front of him at all times, it's very likely this boss will not parry haste. Parry hasting or not, I suspect this will be the gear check gate of Icecrown Citadel. If your three tanks can weather the Saber Lash, you're ready to do Icecrown. If they can't, you need to take a step back and farm the Coliseum a little more. This could be another reason why we're seeing a lot of kills on the PTR. Premades decked out in Best in Slot gear.
The achievement specific to Lord Marrowgar is Unboned, and he's also one of the bosses necessary for Storming the Citadel.
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.