Health:

• 25 Man:  43.2M
• 10 Man:  10.458M
• Enrage:  5 mins

Abilities:

• Inhale Blight - Inhales the Gaseous Blight in the room, increasing damage dealt by 30%. 3.5 sec cast, 15 sec cooldown
• Vile Gas - Inflicts a Vile plague in targeted area, inflicting 4875 to 5125 damage every 2 sec for 6 sec. The plague causes the infected targets to vomit uncontrollably inflicting 3900 to 4100 damage to nearby allies. Instant
• Gaseous Blight - The Gaseus Plague inflicts 2925 to 3075 Shadow damage to all nearby players. Instant
• Gas Spore - Unleashes a Gas Spore, inflicting 2-4 random targets with a gaseous spore. The Spore will explode after 12 seconds, inflicting 1950 to 2050 damage to all nearby friends. The damage from the Gas Spore builds the targets immune system, giving them a resistance to the blight. Instant, 18 sec cooldown
• Pungent Blight - Violently releases the Gaseous Blight, dealing 48750 to 51250 Shadow damage to all enemy players, releasing the deadly Blight back into the room. 3 sec cast, 15 sec cooldown
• Gastric Bloat - Inflicts 14625 to 15375 damage and applies Gastric Bloat to the target, increasing damage done by 10% for 1.67 min and will cause a Gastric Explosion at 10 stacks inflicting massive damage to nearby allies.

The Fight:

For the most part this seems tank and spank gear check.  There's a 5 min enrage, and assuming 5 healers, 2 tanks, and rest dps, you'll need 144K raid dps for about 7.5K avg dps each.

The room will be covered in a yellowish mist (Gaseous Blight) that basically a constant aoe damage to the raid to heal through.   Just pretend it's Saphhiron's aura.

Anyways, throughout the fight, he'll basically start sucking in that gas and increase in his own damage, be wary of tank damage as he does.  You'll probably need CDs for the tanks during the 20 sec after 3rd Inhale.

He's give out Vile Gas, which causes players to puke and hurt others.  So spread out.   Only targets ranged if sufficient numbers.

Gas Spore is something he'll throw out on random 3 people in 25 man (2 in 10 man).  What it does is infect people with Blighted Spores, you want this, every one, including tanks. Spread out after you get it, and Blighted Spores becomes a DoT, heal through that. The DoT also stacks, so ideally you want the spore guys themselves to spread out.    If you should survive, you'll get an Inoculation debuff, which reduces shadow damage taken by 25%, stacks to 3, you want this for the next thing...

Pungent Blight happens after he's sucked all the gas out of the room, well not right away 20 sec afterwards.   Throws it all back out dealing 50K shadow damage to the whole raid, but the guys who got Gas Spore will have a debuff called Innoculated stacks (2 min dura) that will allow them to survive this. Also wipes the debuff off, so have to get Gas Spores again later.

There's also Gastric Bloat he puts on the tank, which increases damage your damage done by 10%, the damge the tank does, not takes.    It stacks to 10, but at 10, you explode, so need a 2nd tank to taunt off at 8 or so.

That's about it, rinse and repeat.

Achievements:

Flu Shot Shortage: - Defeat Festergut while none of the players in your raid group ever had 3 stacks of Inoculated in 10/25-player mode.

This just would mean the raid has to purposely not get the 3rd and heal through the extra damage.

Videos: