This fight has several phases. The first being an Add Phase, follow by a Tank and Spank Phase, and finally a Tank and Spank with Adds Phase. However with about three tanks, prepared DPS, and good healing, you will do fine.
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Kel'Thuzad How To
This fight is basically the same in 10 or 25 man. In 10-Man Kel'Thuzad has 5,660,600 hp. In 25-Man Kel'thuzad has 14,660,000 hp. This fight has a 19 minute enrage timer.
This fight begins when someone enters the green circle in the center of the room. It is recommended that the entire raid enter the circle together. The reason is that Adds will be spawning in the area where you entered from, if people are trailing behind it is likely they will pull an entire group of Adds.
So the entire raid must enter the green circle together. No one should be lagging behind.
Phase 1 of this fight is really simple. During this phase three types of Adds will spawn: Soldiers of the Frozen Wastes (Skeletons), Unstoppable Abominations, and Soul Weavers. During this phase the action is mainly on the DPS. DO NOT LEAVE THE GREEN CIRCLE DURING THIS PHASE. ALL PETS NEED TO BE ON PASSIVE. Ranged DPS needs to kill the Soldiers of the Frozen Wastes (Skeletons) and Soul Weavers before they reach the green circle that the raid is standing in. The Soldiers of the Frozen Wastes (Skeletons) will die very quickly, but the Soul Weavers will take a little longer to kill. The Unstoppable Abominations will be the responsiblilty of the Tanks and Melee DPS. When the Unstoppable Abominations reach the green circle the Tanks should pick them up. Melee DPS then needs to burn them down quickly, if possible Ranged DPS can help with these once they are picked up by the tanks (Personally I use my pet on these once the tank has them).
Phase 2 Positioning
Phase 2 will begin at 3 minutes and 48 seconds after the Phase 1 has begun. At this point Kel'Thuzad will enter the fight. This is a striaght forward Tank and Spank phase. During this phase Ranged DPS and Healing needs to be on the lookout for Shadow Fissure which appears at a swirling red graphic around a player. If you get it just run out of it immediatly and it won't affect you. During this phase Kel'Thuzad may use Chains of Kel'Thuzad on a player and mind control them. These players will need to have Crowd Control (sheep, fear, ice trap, etc.) placed on them until it passes.
The Main Tank should immediatly pick him up and position him at the North point of the green circle (see diagram and video). The main tank should be facing north, and Kel'Thuzad should be facing south towards the green circle (see diagram and video).
Melee and Ranged DPS can leave the green cicle at this point and kill off the remaining Adds. Once all Adds are down melee should position themselves at the 10 o'clock and 2 o'clock postion of Kel'thuzad and stack up (see diagram and video). Ranged DPS should spread out and keep 10 yards (VERY IMPORTANT) between each other. If you have the Deadly Boss Mods addon you can type /distance to bring up the distance window to help you position yourself. Ranged DPS and Healers need to be on the lookout for Detonate Mana. If anyone gets it just be sure your are 10 yards away from everyone else and it will not affecet you or the raid.
Standard raid healing during this phase with one exception. During this fight Kel'Thuzad will use Frost Blast on a random player. This will place an Ice Block on the targeted player and all players within 10 yards of them. It is EXTREMELY important you throw a lot of heals on the players that gets this or they will die very quickly.
Phase 3 - Off Tanks Needed Here
Phase 3 will begin when Kel'Thuzad is at 45%. He will call for assistance and Adds called "Nerubian Crypt Lords" will enter the fight (10 Man - 2 Adds, 25 Man - 4 Adds). When these Adds enter the fight Off-Tanks will need to pick up 1 each on 10 man, or 2 each on 25 man. The remainder of Phase 3 is just Tank and Spank. If you've made it this far you should be able to kill him with no problems.
In Phase 3 be ready to heal the Off-Tanks when they pick up the adds.
Raid Note For Warlocks
If you run out of mana on this fight don't life tap. Instead use Drain Mana on Kel'Thuzad. Because he has a huge mana pool, Drain Mana will replenish your Mana super fast. You can see me use it in the video.