So now you are ready and heading in for your first try at Karazhan. The first boss that you will face is Attumen and his boss Midnight. Lets get down to business shall we:
Trash Mobs Pulls
Undead and Undead Horses - As a tank you might be required to tank 1 or 2 so be ready for it. The biggest concern about tanking the trash mobs is POSITION. Alot of the mobs will be CC either by a priest shackle or at the very least a hunter trap. Another thing to consider will be that certain mobs FEAR. Pull back to the group get your priest to fear ward or have your shaman put up tremor totems to counter the fear. The undead horses does a random charge, so we have a boomkin or a hunter stay far enough from the group to take the charge.
There are a bunch of strategy guides out there but I want to focus more on my experiences to simplify things. You are either going to tank Midnight the horse or tank Attumen for the fight. If you have a feral OT/MT, I suggest he tanks Midnight the horse.
Midnight - just tank him where he stands. He doesn't aggro until you start the fight so don't be afraid to get comfy close with him. If your tanking the horse, your main job is to hold aggro and spam shield block. Cycle through your abilities and make sure to keep an eye on dps people trying to out aggro you.
Attunment - he will spawn at 95%, ideally you will be generating rage by attacking midnight at the start of the fight. This will give you enough to to start with Attunmen once he spawn. Make sure to keep an eye out for him and stand in between the group and Midnight. Make sure to pick him up as soon as possible or else he might one shot one of your squishes. I tend to Auto-Attack on Midgnight so I can focus on Attunment spawning rather than risking him getting away from me.
Once you have him, you will need to make sure you Disarm him as soon as you get a chance. It will radically decrease his melee damage output. You will be on your own as the dsp group will try to take out Midnight first, depending on your dps output. You will want to minimize the damage you take so make sure to spam shield block and disarm him as much as possible.Using Demoralizing Shout might help mitigate damage too.
There is a reflectable AOE curse that he does but I wouldn't want to waste rage or waste your attention on it. Just make sure your druids and mages decurse it off you as soon as possible.
As soon as they MERGE, the aggro table resets. DPS group should stop. Pop Bloodrage. Shield Slam him as soon as possible. This is where you need to pump alot of aggro to grab his attention or else he will kill the group. He does a random charge so we always have a boomkin/hunter stay in front of the group to take the charge. Aside from that, this fight is simple. Disarm, disarm, disarm will be something that you need to rememeber to do everytime you get a chance.