Cho'gall is a two phase boss, the second phase beginning at 25% and lasting until he's dead.
Throughout the fight you have a corruption bar, at specific points your corruption bar is filled you will gain a potentially raid damaging ability so It's important to keep you're corruption bar low by dodging the multitude of attacks Cho'gall uses to increase it.
The Corruption Bar!
A pretty annoying bar that made me move my grid to see it, if you have a really custom UI there are plenty of addons out there that allow you to move the bar (also the sound meter from the Atramedes encounter)
As I mentioned this bar, depending on the % filled of it gives you different abilities.
For every point of corruption old god damage against you is increased by 3%
At 25% you gain a magic debuff that causes you to gain 2 stacks of corrupted blood per second, this must be dispelled
At 50% corruption you begin to channell Corruption: Sickness which deals damage in a frontal cone from you, if you get this make sure to stay away/turn you back against raid members
At 75% you gain Corruption: Malformation which makes a creature on your back cast shadow bolts to nearby raid members, so just move away unless you're a melee dps/tank.
At 100% you become fully corrupted receiving 100% less healing however 100% increased damage and all spells become instant cast
Phase One (100%-25%)
This phase is pretty simple once you understand the incredible amount of abilities going on in the fight.
One of the things that should become fairly evident as soon you engage Cho'gall is his Conversion ability. This ability will "convert" 2 people (5 in 25 man) that begin to channel a spell to Cho'gall. This spell MUST be stopped, they can be interrupted just about any way imaginable, normal interrupts, fears ect. If this isn't interrupted fast Cho'gall gains Twisted Devotion which increases damage dealt by him by 10% each stack, lasting 20 seconds.
Periodically throughout this phase Cho'gall will summon a Corrupting Adherent, they need to be killed fast and away from the raid: They spawn at the portals either side of the room, the first spawning at the left, then right, then left. They need to be killed fast because if they are left alive too long they gain Festering Blood which is a nasty AoE. They must be killed, preferably far away from Cho'gall. The adherents only major ability is their Corrupting Crash which is the same as Vezax's shadow crash ability and is noted by black lines rising out of the ground. Make sure to move from them as they cause corruption!
In this phase Cho'gall will sometimes use two spells that do different things:
-Flame's Orders (seen by Cho'gall absorbing a fire elemental)
Cho'gall deals an extra ~20,000 damage per swing so make sure the tank is topped. He will also spawn fire patches on the ground similar to the fire patches from the Dragon boss in Stonecore, or that the adds on the Beauty encounter in Blackrock Caverns do. These are easy to dodge and should be avoided to prevent raid damage and corruption.
-Shadow's Orders (Seen by Cho'gall absorbing a shadow elemental)
All this does is add a shadow AoE to each of Cho'galls strikes, it deals about ~15-20k damage and during our kill it only ticked about 3 times each time he used Shadow's Orders.
Throughout phase one and two Cho'gall will cast Fury of Cho'gall which will put a debuff on the tank that lasts 45 seconds which increase damage done against the tank, so be sure to tank swap
The most challenging thing during phase one is the adds that spawn from the blood of the adherents. When Cho'gall casts Festering Blood, he spawns five Congealed Blood of the Old God with 75,000 hp (300,000 in 25 man). These will add coruption to whoever they attack so they should be kited, preferably they should be killed/frozen/slowed on the spot and nuked down by DPS as fast as possible. How long phase one lasts increases the amount of Adherents spawned and therefore the amount of blood spawned, so this acts as the soft enrage to the fight.
Phase Two (25%-100%)
In this phase Cho'gall becomes fully corrupted by the Old Gods and goes a bit mental. He fills the room a black AoE that increases corruption and deals damage every 2 seconds to everyone in the room.
He will also spawn four Darkened Creations that are tentacles, 4 in 10 man (10 in 25 man). These should be killed ASAP as they channel Debilitating Beam which reduces healing and damage done by 75% and do alot of damage.
Heroism and all other cooldowns should be saved until this phase as the longer it goes on the more damage you take and I'm sure the healers won't be happy about that
Video:
