http://www.wowwiki.com/Karazhan
Attacks and abilities
Attumen
Basic Melee: 1,600 - 2,100 on tank, 8,000 on cloth.
Immune to taunt and bleed effects.
Shadow Cleave: Frontal melee-range AoE attack for around 4,000 shadow damage.
Intangible Presence: A reflectable AoE curse cast every 30 seconds that reduces chance to hit with melee and ranged attacks and spells by 50%. The area of the curse is centered on Attumen's current target.
Mount Up: When either Attumen or Midnight are at 25% he will mount up, both turning into one boss with the higher health percentage of both.
Berserker Charge: A charge that does 200 damage on cloth and applies a knockdown for a brief stun. It hits a random target at 8-40 yards. This charge frequently does only around 200 damage, it is possible for Attumen to get an attack on the charged target causing 6,000+ damage.
Vulnerable to Disarm.
Attumen's steed, MidnightMidnight
Basic Melee: Around 1,800 damage.
Immune to taunt and bleed effects.
Strategy
At the start, Midnight is alone. When she's at 95%, her master Attumen spawns, right at Midnight's spot. When either reaches 25% health, Attumen mounts Midnight and gains the Berserker Charge ability.
Preparation
Before pulling trash, assign a tank for Midnight and another tank for Attumen. It's best if the Attumen tank is a warrior, to easily disarm Attumen. The Midnight tank is best one that can swap to damage-dealing after Attumen mounts up. Arms warriors or feral druids can typically do this. (It's also possible for a well-geared paladin to tank both Attumen and Midnight simultaneously.) Assign healers to these two tanks and assign a third healer to top off whoever needs it.
Quickly clear all the trash in the stables area. Kill the Spectral Stable Hands first, as they have a heal spell. Control the other trash with Freezing Trap or Shackle Undead. You have only 25 minutes to clear all the trash, so keep moving. If anyone dies during a trash fight, don't wait for a resurrection. Run back in and re-join the fight. Keep pulling trash as long as two healers have mana and the tanks are fully healed.
Phase one - Midnight
Phase One starts when Midnight is attacked and lasts until she is reduced to 95% of her health. She will not attack until she is, herself, attacked. She calls any respawned undead horse trash to assist her, and initiates a basic heavy melee attack.
Phase One is a brief tank-and-spank. The only issue is making sure that the damage-dealers don't pull aggro off of the tank. Simply allow the tank a few moments to build threat, then attack with light damage output. The Attumen tank can build rage by attacking Midnight with white damage.
If Midnight calls any trash, put the Attumen tank on the trash and kill it before initiating Phase Two. Better yet, clear fast enough that there is no such trash.
Phase two - Midnight and Attumen
Phase Two starts when Midnight is reduced to 95% health. It lasts until either Midnight or Attumen is reduced to 25% health.
Attumen spawns near Midnight and begins attacking immediately. He uses his Shadow Cleave and his Intangible Presence curse.
The main danger during Phase Two is right at the beginning. A healer can get aggro and die before the Attumen tank establishes any threat. This generally leads to an eventual wipe. If this happens, run out of the Stables area to reset the fight. Quickly run back in, re-buff, and re-start the fight before respawns start.
When Attumen spawns, healers momentarily stop healing until his tank picks him up. The Attumen tank instantly uses non-targeted threat generation, such as Bloodrage, Thunderclap, Demoralizing Shout, or Consecration. A hunter can use Misdirection to help the Attumen tank (cast it during Phase One). Once the Attumen tank has aggro, he leads Attumen a short distance away from Midnight, and faces him away from the rest of the raid, so his Shadow Cleave can only hit one target.
Any warriors in the raid Disarm Attumen and use Spell Reflection to bounce the curse onto Attumen. Both of these tricks weaken Attumen considerably. Mages and druids remove the curse from the tanks and any non-tank warriors, to ensure that threat generation abilities and disarms land.
Damage dealers, simply hit Midnight without pulling aggro, all the way down to 25%. Healers, conserve mana as possible and use potions. It's a long fight.
Some raids choose to also damage Attumen during this Phase, so that he has little more than 25% health on entering Phase Three. This results in more time with Midnight's melee and less time with Attumen's Berserker Charge.
Phase three - Attumen
Phase Three begins when either Attumen or Midnight is brought down to 25% health. It ends when Attumen dies.
Attumen mounts Midnight and the two of them fight as a single opponent. If Midnight has more health than Attumen, Attumen's health percentage goes up to Midnight's. Attumen gains the Berserker Charge ability, and continues using his Shadow Cleave and Intangible Presence abilities.
Attumen wipes threat at this transition, so everyone stops healing and attacking until the tank establishes aggro. Hunters can help by misdirecting to the Attumen tank.
The Attumen tank again faces his target away from the rest of the raid. Everyone except the Attumen tank moves immediately behind Attumen, so they cannot be hit by the Berserker Charge or the Shadow Cleave.
The Berserker Charge has a minimum range of eight yards. Even hunters can attack from less than that range, so nobody needs to get hit with the Charge. If Attumen manages to Charge someone anyway, he returns directly to the tank afterward, so the tank should not move. Attumen does not use his Shadow Cleave until he reaches the tank, so it's safe for him to run back through the rest of the raid after the charge.
The Midnight tank switches to a damage dealing form/stance and weapon and attacks Attumen.
Warriors continue to use disarm and spell reflection. Druids and mages continue to remove the curse.
The main threats associated with Phase Three are pulling aggro away from the tank (stay below 100% on a threat meter) and the healers running out of mana (bring lots of potions). Generally, however, this is an easy phase of an easy fight.
Post fight
Once Attumen is down, clear the mobs near Koren. He is required for quests, and characters with at least Honored reputation with the Violet Eye can use him to repair and purchase Blacksmithing recipes.