Stage One: My Minions Are Endless!
The room consists of an arena floor area and a couple upper platforms. The majority of the raid stays on the arena floor while a few players will run a mini-gauntlet on the upper platforms.
Any player in these upper platforms will take ~2,000 magic damage every 2 seconds (Unclean Contagion). A golden sphere (Power Matrix) will spawn in the upper platform and must be ran down a path and deposited into a door three times to trigger the second stage.
Every time a player initially picks up the Power Matrix, they receive a debuff preventing them from initially picking up another one for five minutes. Holding onto the Power Matrix reduces movement speed by 20% every 2 seconds. Players can throw the Power Matrix to one another up to 20 yards away, but if it is not caught, the Power Matrix will be destroyed and the process must start over.
Amorphous Cysts are scattered along the paths and spread moss which deals ~2,000 magic damage every 1.5 seconds and reduces movement speed by 75%. Attacking the Amorphous Cysts will temporarily clear the moss. A team of one healer and two highly mobile DPS seem ideal for these upper platforms.
G'huun is invincible until the three Power Matrix are deposited, so the rest of the raid will must deal with adds on the arena floor until that happens.
A random player will receive this debuff and explode after 5 seconds, dealing ~49,000 magic damage to anyone within five yards. Five Virulent Corruption, which look like spheres/spores, will spawn from the affected player and shoot across the room. Any player hit by these will take ~32,000 magic damage and receive a new Explosive Corruption debuff.
On debuff expiration, a little grey circle (Blighted Ground) will also drop under the affected player and anyone standing in it will take ~24,000 magic damage and have their movement slowed by 70%. The affected player just needs to run out quickly and as far away as possible until debuff expiration, while the raid must dodge all Virulent Corruption.
The raid must deal with three types of adds until G'huun activates.
1) Blightspreader Tendril
Casts Decaying Eruption, which deals ~60,000 magic damage to the entire raid, so must be interrupted. Slowly loses energy and upon depletion, simply burrows to a new location.
2) Cyclopean Terror
Casts Torment, which deals shadow damage to a random player every 3 seconds for 15 seconds. This should also be interrupted while these adds are cleaved down.
3) Dark Young (TANK)
The only add that needs to be tanked. Will occasionally cast Massive Smash, which deals ~222,000 physical damage to and knocks back the tank. At 100 energy, will cast Dark Bargain, which creates a circle underneath the itself that explodes shortly after. Any player or enemy hit will have 25% increased damage taken, damage done, and healing done for 30 seconds. Any player that gets two stacks of this will be permanently mind controlled, so players must make sure not to get hit again until the first stack expires. Tanks should hit as many adds as possible with Dark Bargain.
When three Power Matrix are deposited, a Reorigination Blast will occur which deals ~40,000 magic damage to everything, including enemies, every 6 seconds for 24 seconds. The blast also stuns all enemies, increasing their damage taken by 100% for the duration, and clears all Blighted Ground. The raid should stack up and kill any remaining adds before G'huun appears.
Stage Two: Behold the Power of G'huun!
The upper platform needs to be done exactly the same way as the first stage. The team should time the third Power Matrix deposit right as the Gibbering Horror spawns, which is explained later. This will stun the Gibbering Horror along with G'huun with Reorigination Blast, allowing the raid to maximize damage on the two enemies.
The rest of the raid will deal with G'huun and his mechanics on the arena floor.
This is handled exactly the same as stage one. Affected players need to run out quickly to drop their debuff while the raid must dodge Virulent Corruption.
Corrupting Bite (TANK)
While attacking the same target, G'huun's melee attacks deal an additional ~15,000 magic damage per attack, increasing by 15% each attack until the tanks swap. This was extremely overtuned on beta, but it seems like tanks should just swap when damage becomes unmanageable.
Wave of Corruption
A wave shoots from G'huun all around the room, dealing ~18,000 magic damage to all players and applying a stack of Putrid Blood, which deals ~3,000 magic damage every 1.5 seconds. Any players within 5 yards of each other will get hit again, knocked back, and gain an additional stack of Putrid Blood.
Eventually G'huun will immobilize a random player, telegraphed by an orange circle, for 10 seconds. After 10 seconds, all players within the circle will have their Putrid Blood stacks removed, and a Gibbering Horror will spawn. This add receives 5% maximum health and additional damage done per Putrid Blood consumed. It casts Mind-Numbing Chatter, which can't be interrupted and deals ~13,000 magic damage to all players, interrupting them for 3 seconds.
Depending on how this is tuned, this add sounds like it could be extremely strong, so I'm not sure if you can have everyone clear their Putrid Blood at once. Either way, the upper platform team should time their third Power Matrix deposit with the spawning of the Gibbering Horror so it can be hit by Reorigination Blast and burned down fast.
Stage Three: Your Destruction Is Assured! (15%)
I'm not sure if anyone got to this stage on beta, but it seems like stage two continues until G'huun is at 15%, when he destroys the upper platforms. This will cause debris to fall from the ceiling, which deals ~305,000 magic damage to all players, decreasing the farther away players are from the point of impact, and also knocks back. It's unclear whether this happens once or continually, but players need to get really far away from wherever the impact is.
This is handled exactly the same as stage one and two. Affected players need to run out quickly to drop their debuff while the raid must dodge Virulent Corruption.
Corrupting Bite (TANK)
This is handled exactly the same as stage two.
Wave of Corruption
This is handled exactly the same as stage two with players remaining spread, except the Putrid Blood stacks can't be cleared, so it acts as a pseudo enrage.
G'huun creates a series of these growths and they explode, dealing ~104,000 magic damage to anyone hit within 8 yards. This sounds like circles will spawn on the ground and players will have to dodge them.
Gaze of G'huun
G'huun shoots out a wave that causes all players facing him to be feared for 6 seconds. I'm guessing this is telegraphed in some way and players just need to turn away for it.