Anub'arak is a reasonably well put together boss encounter. It is a bit like Anub'rekan in Naxx.
The fight has three phases - in P1 the MT pulls the boss and everyone starts hitting him. He does the same front cone AoE that he did in Azjol Nerub when you fought him, so face him away from the raid. You don't need to tank him in any special place or on top of the permafrost (see below for an explanation of permafrost). Throughout the fight he casts Penetrating Cold, a DoT. That is only applied to the tank in 10 man, so should not bother you much - just heal through it.
After a while he will spawn an add. The second tank should pick this up and ranged dps should nuke it down. Melee stay on the boss. The adds can be stunned. Every so often the add will try to submerge - if they succeed in doing so then they will come back a while later (usually when you have one or more other adds up) and there's a danger of being overwhelmed. You can stop the adds submerging by stunning them, or tanking them on top of the permafrost. I recommend the latter, but if you are unsure of your positioning then it's no bad thing to stun them just in case.
There are blue globes on the ceiling - someone needs to shoot these down (probably a hunter). When they hit the ground they spawn a patch of ice called permafrost. Assign someone to do this throughout the fight.
After a while a second add will spawn. Offtank this for a while, and leave your dps on the boss.
After a little while longer the boss will submerge and you will enter P2. You cannot stop the boss from doing this by positioning him over permafrost.
At the start of P2 your dps should kill the remaining add. The little bugs around the room now turn hostile and start attacking players. They need to be killed. If you are the tank resist the temptation to help! The adds place a stacking DoT effect on their target, and each new application refreshes its duration. If you try to tank the little mobs you will be left with a large unpleasant DoT effect. On our first try when I attempted to AoE tank the bugs I had so many applications of the DoT effect that I was taking 3000 dps from it.
In P2 Anub (who you will remember is currently underground) will periodically chase members of the raid, like one of those worms in the film Tremors. He will fire spikes into the air while he does so. The chased raid member must run away from Anub as he does this onto a patch of permafrost. Anub will try to emerge from the ground and impale his target, but will be unable to do so and will be stunned. Dpsers not being chased should nuke down the little bugs in this phase. Healers should regen a bit if they can.
After a while Anub will resurface and you are back into P1 again. It's a good idea to try and limit Anub to just one burrow phase, so if you are below 65% at the beginning of the second P1 it's a good idea to ignore both of the crawler adds in the second P1 and just offtank them for the rest of the fight. It's worth noting that the adds increase each others' damage as well as applying an unpleasant DoT to the tank, so prepare for more than doubling of the incoming damage on the offtank here. If The boss' health is above 65% at the start of your second P1 then you are unlikely to be able to push him into P3 in that phase and will have to resign yourself to another burrow phase.
Once he hits 30% Anub stops bringing in adds, and will no longer burrow. Result! Sadly he now casts Leeching Something. This drains 10% of the raid's health per second, or 250 health per person, whichever is more. Damage done heals the boss. The strategy here is that the only people taking other damage should be the two tanks. These need lots of heals in this phase, and both tanks may wish to start chaining cooldowns, pots and trinkets to increase survivability. Other raid members should not be taking damage, and healers should keep them at about 30% to 50% to minimise healing done to the boss - the raid heals are just necessary to stop you dying in about 5 or 6 ticks of the Leeching Thingy.
Save heroism and other dps cooldowns for P3 and he goes down pretty fast.
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