Tactics: Attumen

Strategies
At the start, Midnight is alone. When she's at 95%, her master Attumen is summoned, right at Midnight's spot. When either reaches 25% health, Attumen mounts and gains the Charge ability.

Preparation
Clear the whole area, because all trash mobs will come when you pull Attumen, save for the five next to the blacksmith. Trash has a 25-minute respawn, so a little speed is required during the clearing phase. Respawns will not happen during the boss fight.

Tanking
One tank should start the encounter by pulling and tanking Midnight. Your off tank (preferably one that can Disarm) will be waiting between your group and Midnight. When Attumen spawns, he should immediately be picked up by this tank and drawn away so that Attumen's Cleave doesn't hit the Midnight tank. One effective way to do this is to activate mob health bars (default keybind V) and Thunderclap immediately upon seeing Attumen's healthbar appear. Follow with a Demoralizing Shout then and a Disarm, as quickly as possible, and have a Hunter use Misdirection on Attumen onto his tank. A Paladin tank can use Consecration and Exorcism to pick up Attumen as soon as he spawns (Level 1 or 2 Consecration left on ground under Midnight). Disarming Attumen will reduce his DPS by about 60%, at which point he'll be less damaging than Midnight. Also, a warrior can use Spell Reflection before Attumen applies the curse, which is triggered every thirty seconds.

Midnight should be tanked well away from the raid. Tanking her where she stands is preferred.

Charging
When either horse or rider are at 25% health, Attumen will mount. Aggro will be wiped, and if he is below Midnight's health, Attumen will be healed up to the horse's percentage. Since aggro is reset when he mounts, it's a good idea for the tank (if a warrior) to use Shield Wall at that time, to give the healers some slack. This will allow the warrior to build threat again, without requiring significant heals. In the case of a Paladin tank a quick shot of Exorcism, judging Righteousness or Vengeance should allow DPS to come in with very little gap for threat generation.

The only difference after mounting is the additional charge ability. It is not entirely clear how the targeting of the charge works. One theory says that it hits the furthest person who has no aggro, so that it would be advisable to put a plate / mail user at the back of the raid. The other theory is that the charges are either random or target the nearest non-melee target, thus the off-tank should be about 8 yards from the boss.

Keep in mind that he can and sometimes will use his shadowcleave while moving back to the MT after a charge.

The best way to avoid damage to the raid is by having the tank turn Attumen around, and having everyone in the raid stand directly behind him between 6 and 7 yards away (outside cleave range, but too close for the charge).

DPS
Midnight should be focused on first. It is possible to switch DPS to Attumen when Midnight gets down to about 30%, so that Attumen has low health when he mounts up. This shortens the length of the phase where he can charge, although it requires tanking Midnight for that much longer.

Healers
Healers should keep everybody's health topped off through the whole fight, especially in Phase 2, because the charge can hit anybody if the group is not properly oriented. Charge victims should bandage to lessen the healer load.

Curse
Mages and Druids should focus on removing the debuff from everyone.

If there's a warlock in the raid, Curse of Weakness could be used on both Attumen and Midnight. This reduces their attack power by up to 350. However, it does not stack with Demoralizing Shout and prevents the warlock from using a different curse. If a warrior is present, using Curse of Weakness will in effect be a waste of Demoralizing Shout.