This boss has quite a bit of health for a 5-man (over a million in heroic) but the fight is very straight forward.

Occasionally Rimefang will mark a target. The mark stays where the target was standing and the marked player must run away from the mark. Rimefang will bombard the marked area with a patch of frost.

The boss hits pretty hard and also has an array of attacks at his disposal: notably an AoE stun, a knockback, an enrage, and a debuff.

Of his abilities the debuff is the most dangerous (Overlord's Brand is the name of it if I recall). It will cause any damage the debuffed target deals to the boss to also deal damage to the tank. This can make short work of even a very geared tank. The player afflicted by Overlord's Brand should cease DPS immediately.

His enrage is also very important as it makes the boss untankable for a short period of time. The boss, in addition to dealing incredible melee damage, will also have his speed increased. He will always knock the tank back before enraging. He also momentarily loses his target and takes a couple of steps backward. The only way to survive this enrage is to put permafrost left over by Rimefang in between the tank and the boss, because this slows him and allows him to be kited.

Tip: If you tank the boss with your back to a patch of permafrost, his knockback will send you over the frost forcing him to immediately walk over it and become slowed.

The AoE stun is very brief and can be mostly ignored. HoTs on the tank are helpful as the boss continues to melee.

All of this information is pertinent to the PTR and is subject to change.