Kel'Thuzad
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Overview
When it came to spreading the Plague across Lordaeron, Kel'Thuzad was one of the Lich King's main agents. The former sorcerer of Dalaran, who was slain by Arthas during the Third War and was reborn a lich in order to do his bidding, now reigns over Naxxramas.
Although this fight technically has three distinct phases, most of the raid members will no notice the change between phases 2 and 3 since it doesn't necessarily affect them. The main key to Kel'Thuzad is self-awareness and awareness of your surroundings.
Gear and consumables
Like most Wrath of the Lich King raid encounters, this fight is tuned for players who are using level 80 epic gear, as well as the best flasks and food buffs available. Getting the best possible gear for your character will make this fight a whole lot easier.
In WotLK, professions can be used to craft high quality epic gear. Most raid bosses also drop an epic bind-on-equip item. These items are the easiest way to get your character geared up fast, but they're expensive to craft or buy on the AH. The easiest way to get your hands on them is to use a guide like WoW Gold Secrets, which has tips for making several thousand gold per day. This is a great way to save time and down bosses faster.
Click here to try out WoW Gold Secrets.
Abilities
HP:
Regular - 5,229,400
Heroic - 14,640,000
Phase One
Soldier of the Frozen Wastes: The Soldiers are skeletons that make a beeline directly for the raid. They cast Dark Blast when they touch a raid member, causing 3,000-4,000 shadow damage to the raid and detonating the Soldier. Even with Shadow Ward, Dark Blast is not partially resistible. Soliders can be shackled although it is rarely necessary. They walk slowly and only have a little more than 4,000 hit points (often one-shotted by ranged DPS). Your raid will deal with approximately 70 Soldiers before the first phase is over.
Soul Weaver: The banshee Soul Weavers feature Wail of Souls, which inflicts between 6,000-9,000 shadow damage and a knockback that can knock three players 30 yards backward. With their extremely slow movement speed it is easy to burn them down before they are within range to do the cast the knockback. You will contend with less than 10 of them at 150,000 hit points each.
Unstoppable Abomination: The Stitches-looking Abominations apply Mortal Wound, a -10 percent healing debuff on whatever it is attacking. These are the only mobs that should be allowed to reach the raid and should be tanked and burned down quickly. All melee DPS should focus on killing them. They have approximately 210,000 HP and fewer than 10 will attack during Phase 1.
Phase Two
Chains of Kel'Thuzad: In the Heroic version of the encounter, Kel'Thuzad can mind control three people at once. When mind controlled, their size is increased by 200 percent, their damage is increased by 200 percent and healing is increased by 500 percent. Rather than have them buff and heal Kel'Thuzad or attack raid members, crowd control them using any method: polymorph, ice trap, hex, etc. Mind Control lasts 20 seconds and cannot be dispelled.
Frost Blast: Places an Ice Block effect on the targeted player and all players within 10-yard radius, rendering them unable to perform any action. While the player is frozen, the frost block will deal 104 percent of their maximum hit points in four seconds. Frozen players will not survive if they do not receive some healing during the process. In the Heroic version, the main tank can be frozen unlike the 10-man.
Frost Bolt (Multiple): Hits everyone in the room every 15 seconds for 4,000-5,000 Frost Damage (10-man, 7,000-8,000 in Heroic) and slows movement speed for four seconds. The multiple Frost Bolt is fully resistible.
Frost Bolt (Solo): Unlike the multiple Frost Bolt, which is instant cast and regular, the solo Frost Bolt has a two-second cast time and occurs randomly. In the 10-man, the solo Frost Bolt does approximately 10,000-13,000 damage and theoretically be healed through. However, in the Heroic the solo Frost Bolt hits for approximately 30,000 and should be interrupted by Rogues and Warriors.
Mana Detonation: A debuff randomly applied to a mana-using player. The player explodes after five seconds, dealing Arcane damage based on their maximum mana to all players within 10 yards. This damage does not hit the player itself. Anyone who has been hit by the Mana Detonation debuff should run away from all other raid members.
Shadow Fissure: These summoned red fissures are focused on a player and stew for five seconds before doing between 50,000-140,000 Shadow Damage. Because of the 5-second warning to move, no one should ever be hit by the Void Blast.
Phase Three
Guardians of Icecrown: At 45 percent health, Kel'Thuzad calls for aid and is assisted by two (in the 10-man, four for Heroic) Nerubian Crypt Lords known as the Guardians of Icecrown. Each Guardian has HP equal to Kel'Thuzad and has a Blood Tap ability which ups its damage and size and stacks.
Preparation
Raid composition (10 man)
2 Tanks
3 Healers
5 DPS
Raid composition (Heroic)
1 Tank
1-2 Off Tanks (to contend with the Guardians in Phase 3)
6-7 Healers
15-16 DPS
Assignments
The following macro describes example raid assignments:
/rw Kel'Thuzad Incoming
/raid [Tank 1] MT
/raid [Tank 2] tank left Guardians Phase 3, [Tank 2] tank right Guardians Phase 3
/raid Call out if you are Ice Blocked at any time
On the Pull
The entire raid should run into the center of the room at the same time and stand within the confines of the circle in front of Kel'Thuzad. If anyone is left behind or moves too slowly, you run the risk of aggroing an entire room of adds, instead of just dealing with them piecemeal.
Phase One
Upon engaging Kel'Thuzad, he will remain invulnerable for three minutes and 48 seconds while you battle with his minions, which spawn in the seven cubbies around the room.
Ranged DPS Focus on the Soldiers of the Frozen Wastes and the Soul Weavers, burning them down before they reach the raid. Ranged should also keep their pets and minions set to passive so they do not run in to attack and aggro any of the cubbies.
Melee DPS, 
Tanks All melee should focus on the Unstoppable Abominations when they reach the group with the tanks tanking them.
Phase Two
After 3 minutes and 48 seconds, the rooms will despawn and Kel'Thuzad becomes active, beginning Phase 2. The Main Tank should be ready at this point to pick up Kel'Thuzad when he becomes active.
Main Tank Kel'Thuzad should be tanked halfway between his Throne and the circle on the floor, providing room for melee DPS to fit in and leaving space for the remainder of the raid to spread out comfortably.
Ranged DPS If there are any Soul Weavers or Soldiers remaining, they should be burned down by ranged melee.
Melee DPS If any abominations remain, they should be burned down by melee. Once the adds are burned down, it is time fore melee to head toward Kel'Thuzad. Because there is the 10-yard limitation thanks to Frost Blast splash, it is important that the melee know their position around Kel'Thuzad before running in.
Picture a diamond formed around Kel'Thuzad: Main Tank standing at his front, one melee on each of his sides and one behind him. All of them should stand as far back as they can while still being able to hit him.
Should you have an overwhelming number of melee, it is best to stack them in groups of two in the diamond pattern, it is better to just have to heal two through the Frost blast than four or more because they were bunched up or spread out evenly.
Raid Once all the adds are down, focus fire on KT. The entire raid should be spread apart, yet in range of the healers. General rule of thumb is at least 10-yards away from everyone else, to avoid Frost Blast splash and Mana Detonation damage. If you have been targeted by Mana Detonation, it never hurts to run further away if possible - it is better to be safe than sorry.
The basic premise of the fight does not change between Heroic and Regular, however, the Heroic version tends to be much more hectic with more people getting Frost Blasted and Mind Controlled. In Heroic it may be more useful if raid members call out when they are Ice Blasted so they receive the heals they need to survive.
Raid members should also call out when they are Mind Controlled. Any one who has CC abilities should shift their focus to CC-ing the Mind Controlled raid members before they have the chance to buff or heal Kel'Thuzad or harm raid members.
Rogue, Warrior Rogues and Warriors alike should stay on top of Kel'Thuzad's solo Frost Bolt and interrupt it whenever possible. The solo Frost Bolt has a two-second cast time and hits for 30,000 in Heroic and 10,000-13,000 in Regular.
Raid Leader You may choose to have raid members call out when they are Frost Blasted (especially in Heroic, when it happens to multiple people at once), to help the healers top them off more quickly. The same goes for Mind Controls in the Heroic version. While it is very obvious when it is happening, it may help to know who the MC'ed targets are in order to speed up their containment (especially if they are one of the regular Crowd Controllers).
Healers After healing through Sapphiron, this battle shouldn't give you too much trouble, just remember there are many more variables to be dealing with. After the multiple Frost Bolts you will need to get everyone topped off again. Your biggest challenge will likely be in keeping Frost Blasted players alive since you only have four seconds before their imminent death.
Phase Three
Offtank, Healers Upon Phase Three, a tank should be prepared to pick up the Guardian adds (two tanks for the Heroic adds). The adds should be tanked away from the raid and will be ignored by DPS, who will continue to burn down Kel'Thazud. Once he is dead, the Guardians will run away. If possible, designate a healer to focus on the Off Tank(s) for this phase as the Guaridans end up hitting pretty hard as time passes.
Achievements
Kel'Thuzad's Defeat - Defeat Kel'Thuzad.
Just Can't Get Enough - Defeat Kel'Thuzad in Naxx while killing at least 18 abominations in his chamber.
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