Many, if not all of you are taking your first steps into Naxx by now. On most servers, a large number of guilds have successfully cleared it in its entirety at this point as well. I’m writing more for those who are new here, in order for you to better help understand the mechanics of the fights and what to expect so that you can successfully collect your epics! Regardless of what role you have in the fights – tank, DPS or healing – I’d recommend that you read over the roles that each group will be performing in order to have a clear understanding of how the whole fight will go.
Kel’Thuzad
The final boss of Naxxramas. Kel’Thuzad is a fairly easy fight in terms of mechanics and understanding, as the abilities and phases are easily recognized and countered. However, the timeframe allowed to react to these abilities is very short, and will require the entire Raid to react appropriately at the correct time in order to survive them. If everyone can do so, particularly the Healers, then he can be taken down fairly easily with a little bit of luck. Late reactions on just a few occasions, though, will cause Raid deaths and the attrition from these will most likely mean the difference between getting him down and recovering from a wipe to try him again.
To get to him, simply follow the corridor from Sapphiron. You can proceed safely inside the door to his room and onto the dais immediately on the other side; do not get too close to the lip of it or you may risk starting the event. Kel’Thuzad is a three-phase encounter, and as such is better explained through each of those phases.
You will need 2 Tanks, 3 Healers and 5 DPS. Ranged DPS is preferable to Melee; if you have more than 2 Melee DPS, it may cause issues due to his fight mechanics, as explained later.
Kel’Thuzad Phase One
The first part of the encounter is the add phase. During this phase, Kel’Thuzad will not be attackable, nor will he attack. The Raid will initiate this phase by running into the middle of the room until you are all standing in the area designated by the large green circle on the floor. It is important that the entire Raid move together to this point, as the area that you are coming from will spawn a multitude of adds by the time that the first member reaches the circle, and any straggler may aggro them and cause a near certain wipe at worst, or their own death at least.
Once you have reached the circle, Kel’Thuzad’s minions will begin to converge on the party. They will come slowly at first, and in greater strength and numbers as the timer wears down (approximately 3:30 until the Second Phase starts).
Adds will come in 3 forms: Skeletons, Banshees and Abominations. The Skeletons are easily downed as they have very low health. Should a Skeleton reach the Raid, it will detonate and cause splash damage to everyone. They are weak enough that a single cast for most primary ranged spells (Shadowbolt, Frostbolt, Fireball, Lightning Bolt, Wrath, etc) should down them, even if it is a Healer doing the casting. Banshees are stronger and will drift slowly towards the Raid. When one is active, all Ranged DPS should immediately focus fire it down. Abominations will be picked up by the Tanks on their way in and then engaged by the Melee DPS. They stack a Mortal Strike debuff on whomever is tanking them.
Of course, those are general rules. In order to stay on top of the adds, everyone should be doing some element of damage dealing at all times. If adds are downed quickly, Raid damage is fairly minimal and easily handled by one Healer. Thus, the other two Healers should focus their efforts on blowing up Skeletons. Paladins lack a good ranged attack for this, but Shamans, Druids and Priests (Lightning Bolt, Wrath / Moonfire, Smite, respectively) should be able to effectively control the Skeletons that Ranged DPS cannot get to. Similarly, any Melee DPS that is not currently occupied with an Abomination should feel free to make use of their Ranged attacks and assist with the Skeletons when possible as they come in large numbers. If everyone helps out with add control, the phase should go fairly smoothly and not overwhelm the Raid. After 3 1/2 minutes, Kel’Thuzad will enter the combat.
Kel’Thuzad Phase Two
Kel’Thuzad will now join the battle. He must be immediately picked up by the Main Tank once he is active, and tanked somewhere between his initial spawn point and the current Raid position. Some adds will still be active when he spawns; mop them up as quickly as possible and get in position for the fight.
Everyone should now spread out throughout the room. The fight necessitates that all players be at least 10 yard from the next closest player in order to prevent really bad things from happening. This is where a sizable amount of Melee DPS is a hindrance, as (despite a large hitbox) you will not be able to move all players into melee range without endangering the Raid. If you have more than 3 Melee players (and this *is* counting the Main Tank), consider having the balance of them move to a ranged position and contribute whatever they can, no matter how meager. Fortunately, outside of the need to duck in and out of Void Zones, there is almost no movement required by the Raid at this point, so spacing is easy to maintain.
If the Offtank engaging in combat would mean 4 people in melee range for Kel’Thuzad, then that person should also stand clear of him as it is far more important that the Offtank survive into Phase 3 than contribute DPS.
At this point in the encounter, there is no need for a fast burn. The Raid may remain methodical and slowly work his health down until he reaches Phase 3. It is significantly more important that people survive into the Third Phase than it is to burn through his health right now.
Kel’Thuzad’s primary attack in this phase is a Frostbolt which he will direct at his highest aggro target. It is interruptable and has a long cast time. Whomever is engaged in melee, and possibly Elemental Shamans and Mages as well, must work out an interrupt rotation for this cast as it is a very heavy hitter. Properly interrupted, it is a non factor in the fight. Kel’Thuzad should never be allowed to get this cast off.
Periodically, he will launch an AOE Frostbolt at the Raid. It is resistable, and not interruptable. However, it will only hit for about 4-5,000 points and is easily healed through. He casts it infrequently enough that there is no danger of a second one hitting before the targets of the first one have been topped off.
Periodically while fighting him, he will spawn a Void Zone on a player’s location. These are easily avoided so long as the player notices that they are spawning. The Void Zone will take five seconds to manifest, and then disappear five seconds afterwards. If you see one forming on your position, simply strafe slightly to one side, and move back when it is gone. Note that, unlike the Void Zones that you have seen throughout most Raids, these do not play nice and *will* kill you in a single tick should you be caught in one.
Kel’Thuzad will also use an ability called Detonate Mana on anyone who has mana. The ability will cause a Mana Burn effect on the player, and anyone within 10 yards will also be hit with a heavy-damage AOE that is centered on the target. The target of Detonate Mana will not take health damage themselves. Since nobody should be closer than 10 yards to each other, nobody should be taking damage from this. Additionally, the healing needs from this fight are sufficiently manageable that it should not be considered a “soft enrage” attrition mechanic that must be contended with, like Moroes’ Garrotes were. Most targets of the ability will be able to sufficiently maintain mana over the duration of the fight, regardless.
His most dangerous ability at this point is Frost Tomb. It occurs about every 45 seconds; when it does, Kel’Thuzad will encase a random member of the Raid in ice. Over the course of 5 seconds, this person will sustain damage equal to a percentage of their total health each tick; the sum total of the ticks is greater than 100% (I believe it is 105% or 110%, but I’ve heard as high as 130%. It’s noticeable). Thus, if this person does not receive healing immediately, THEY WILL DIE. Frost Tombs cannot be bubbled out of or Iceblocked; supposedly, a Warlock may Sacrifice a Voidwalker while inside to save themselves, but that is unconfirmed. Basically, all three Healers *must* be on constant lookout for this debuff in Raid Frames, and react to it the moment you see it. Stop casting your current spell, and get the fastest 2 heals that you can off on the afflicted party or it’s curtains. If I could point to one single mechanic in the whole fight that makes or breaks it, the reaction time to the Frost Tombs is it.
What makes Frost Tomb particularly nasty is that they chain to other players within 10 yards. This is why limited amounts of Melee is better. Assuming you have Main Tank + 2 Melee, they should set up in a triangle at the outer limits of Kel’Thuzad’s hitbox. If this is done, then only one person at a time will be vulnerable to the Frost Tomb. If Melee is closer together, it may chain to multiple players – including the Main tank – and that will likely be a Raid wipe.
As a Resto Shaman, I basically put together a “Kitchen Sink” macro that may save a Raid in this occurrence. Essentially, I bundled Tidal Force + Nature’s Swiftness + Trinket + Chain Heal into one “break glass in case of emergency” cast. Outside of that ability, though, there isn’t much else that will see you through safely if this happens to your Raid. Better to just avoid it by spacing out well. Additionally, it is wise to have your Raid setup in such a manner that any player may be targeted by all three Healers, so as to better contend with the Frost Tombs. Healers should advise through vent if someone is out of range, so that one party or the other may react accordingly.
Should you have Melee DPS that is attacking from range at this point in the fight due to having too many Melee, they should be ready to move into a spot if a Melee DPSer falls. In an alert Raid, though, that occurrence should not happen. Simply whittle away at Kel’Thuzad for now, countering his abilities and healing through damage, until he reaches the 40% mark. This will be the burn phase.
Kel’Thuzad Phase Three
When Kel’Thuzad reaches 40% health, he will spawn 2 adds, Scarab Lords called “Guardians of Icecrown”. On spawning, the Offtank must immediately pick them up and move to a safe spot near the rear of the Raid, but still in range of all three Healers. These adds have almost as much health as Kel’Thuzad himself and are not meant to be killed. Allow the Offtank to hold them while the Raid burns Kel’Thuzad.
The dangerous mechanic now is that these adds have a stacking buff which increases their damage 15% per stack – and stacks up to 99 times. It will not take more than a few minutes before the damage is unhealable, so the raid is now working against the clock before the big bugs get loose.
Kel’Thuzad’s abilities do not otherwise change during this phase; however, if you have been holding any Melee back, this may be the time to throw them more directly into the fray, space out as best you can, and hope that luck is on your side. Now is the time to use Bloodlust and any other long cooldown abilities that you have been saving up because it is absolutely imperative that he be downed expediently. Depending on the mitigation abilities of your Offtank, you have about 2-3 1/2 minutes before the Guardians of Icecrown get to be too much to handle and wipe your Raid.
Stay on top of your healing, keep a solid damage rotation and maintain good Raid spacing and you should be able to push through it. Recognize, though, that this fight usually has a little bit of luck to it with regard to who gets Frost Tombed and when; much like Prince Malchezaar and his Infernals, even a well prepared veteran Raid can run into a roadblock from time to time. Be patient and execute solidly and you should down him every time.
Too Many Alts 
The boss mechanics do change significantly in the 25-man version.
First of all he gains “Chains of Kel’Thuzad” which resets threat and mind controls 3 random players for 20 seconds, increasing their damage and healing by 200%/500% respectively.
Secondly, frost blast on melee becomes much nastier and you’ll always get multiple melee frozen. His single target frostbolt hits for 30,000 unresisted (~8,000 AoE).
You get four Guardians in phase 3 and they hit considerably harder but your offtanks can be mind controlled as well as frost blasted, meaning 4 tanks is ideal.