Quick Facts:
This is like Gruul's Lair, no attunement needed. The only requirement for this boss is to be level 65 or over and be in a Raid Party to enter his instance.
Magtheridon was formerly serving as the final quest chain for the "trial of Naruu" quest chains which involved attunement to The Eye.
Magtheridon is around the same difficulty as Hydross the Unstable, and fairly harder than Gruul the Dragonkiller.
Prefferably Karazhan and Gruuls Lair need to be on farm, without major difficulties in clearing those, you might stand a chance against Magtheridon.
Most of this fight is VERY dependant on Class Balance and the awareness of your raid.


A quick Overview over this encounter:

Magtheridon have 3 phases where the first part is arguably the hardest part, especially for the healers.
When you enter Magtheridon's Chamber you will see 5 Channelers channeling the Manticron Cubes to Banish Magtheridon. Nothing in here can be aggroed by standing too close. The only way you aggro in here is to perform hostile actions to either one of the 5 channelers or to Magtheridon,
so it is possible to take positions.

Group Setup:
Because of the 5 channelers at the start of the Magtheridon phase you will need 5 tanks and 5 warlocks for an ideal kill, less warlocks will make the encounter harder while 5 tanks are an absolute must for learning phases, can be done with 4 with more experienced raiders.
Bring at least 5 healing classes that exel at single target healing aswell as healing a little on the raid, Restoration Druids, Holy Paladins are very good at this. Last but not least bring very very very high DPS with you, you're going to need it.

Magtheridon

Magtheridon - Marked as ??(73) (Boss)
Hellfire Channeler - Marked as 70. (Elite)

Hellfire Channeler abilities:

Dark Mending:
A very potent healing spell that heals the Channeler or other Channelers for 69375-80625. This spell have a pretty fast cast speed so have warlocks or rogues to use Mind Numbing Poison on them so it is easier to interrupt.

Shadow Bolt Volley:
Hurls Shadow Bolts on nearby raid members, doing 1063-1437 shadow damage, to people in a 30 yard range.

Summon Burning Abyssal:
Sometimes the Channelers will spawn Burning Abyssals, crushing down on a random raid member and dealing around 2625-3375 damage. Burning Abyssals have an Immolation Aura that does fairly decent Fire damage, they Fireblast random raid members for 2700-3300 fire damage, and despawn after 1 minute. A channeler can't have more than 2 abyssals up at any given time. These Abyssals can be Banished and Feared. A maximum of 10 Abyssal can be up on the raid.

Magtheridon's abilities:

Standard Melee:
Hits tanks for around 3000-6000 damage.

Ground Tremor:
Magtheridon makes the ground tremble and shake knocking people around for 5 seconds in random directions. Only Instant cast spells can be used as channeled or casting spells will be interrupted. Magtheridon keeps hitting the tank at this ability.

Conflagration:
Magtheridon will set aflame the ground with a fireball making people that walk through it ignited and disoriented taking periodic Fire damage and damage nearby raid members for 300 fire damage per second.

Cleave:
A frontal cone Cleave that hits for around 8000 physical damage, it's on a 10 second cooldown.

Blast Nova:
Magtheridon will start casting Blast Nova every now and then dealing from 2188-2812 fire damage every 2 seconds for 10 seconds to the entire raid.

30% enrage:
At the 30% marker Magtheridon will in his anger destroy the foundations of Hellfire Citadel, debris will hit the entire raid for 5000-6000 physical damage and knock them down.


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The Manticron Cubes:
The five cubes the channelers use to banish Magtheridon, are needed to stop Magtheridon using Blast Nova. However, Manticron Cubes deal damage over time when you engage them and the channel lasts for 5 seconds. Every Manticron Cube must be activated before Magtheridon gets Banished. When Magtheridon is chained down by the Manticron Cubes he will take 200% damage as a Bonus. If a Banish is succesful the Players using the Cubes will get Mind Exhausted and cannot use the Manticron Cubes again for 1.30 minutes, so you need 2 teams for clicking the Cubes.

It is possible to permanently banish Magtheridon if you have exeptional healers and extremely aware raid members making this fight this much easier, but it's extrememly hard, it's not suggested to try this.
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Strategy:

Phase 1:
Assign one tank and one healer for the channelers, every channeler must be tanked and their heals MUST be interrupted. Every DPS focuses fire on one Channeler and moves on to the next one while the healer assigned to their tank moves on to help the other healers, if his tanks Channeler dies. Everytime one channeler dies, the remaining Channelers gains a stacking buff which increases casting and attacking speed by 20%.
When channelers spawn Burning Abyssals have warlocks to Banish and Fear them until they despawn (1 minute), if there are not enough, bring an offtank to deal with those.
After 1 minute Magtheridon will no longer be banished and needs to be picked up.

Phase 2:
Kill the remaining Channelers and Burning Abyssals and start focusing on Magtheridon. Basic tank and spank until he starts his Gound Tremor, use instant healing spells o damage prevention spells like Pain Supression or Power Word: Shield.
When Magtheridon starts casting Blast Nova have your assigned teams to click the Manticron Cubes and heal them until Magtheridon is sucessfully banished, keep him Banished until the channel wears off for extra damage, if you can afford the health cost.
Rinse and Repeat until he hits 31%

Phase 3:
Stop attacking at 31% and stop dots at 35%, heal the raid to full HP and give the Main Tank HoT's. The main tank should Shield wall at the ingame cinematic when he reaches 30%
When the entire raid have suffered from the shattering of Hellfire Citadel, heal the tank to maximum, then start topping of the raid fast, starting with the tank then the 1st Cube team, 2nd Cube team, yourself then the remaining. After the shattering this is like Phase 2, rinse and repeat and Magtheridon is slain!