Marrowgar
Being the first boss of a new raid, Marrowgar is not an overly complicated fight. This fight requires 2 (25-player: 3) tanks, so that the Bone Slice's cleave damage can be properly distributed among the tanks (like Nalorakk or Koralon the Flame Watcher). He will also target a random player with his Coldflame ability - preferring people at range. Only if everybody is inside his hitbox will the tanks be targeted. This projectile spreads a line of fire in its path which will inflict a decent amount of damage per second to anyone caught in it - simply taking a few steps aside is enough to avoid it.
Throughout the fight Marrowgar will cast Bone Spike Graveyard, choosing one player (25-player: 3) to impale on a bone spike. The player(s) will suffer physical damage, become incapacitated, and lose 10% health for every second they remain impaled. The DPS should kill the spikes as soon as possible.
The last thing to watch out for is the Bone Storm. This spell is similar to Stormcaller Brundir's Lightning Tendrils, only it deals physical damage. Marrowgar will start a whirlwind, and practically charge random players (he moves very quickly), stay at their location for a few seconds (while in whirlwind), then switch targets. The Bone Storm doesn't do a whole lot of damage, especially if you wear mail or plate - get out of it, but don't panic, it's not that deadly. It is also worth noting that the Bone Storm's damage is based on proximity - players close to Marrowgar will take the full damage of the spell, while players who are further out will take significantly less damage. Another trick to mitigate this spell's damage is to move constantly. Since Marrowgar will charge a player's location, not the player himself, always moving will allow the target to put some distance between the boss and itself. Marrowgar will charge total of 3 times during each Bone Storm.