Following its defeat in the Emerald Nightmare, Il'gynoth retreated into the depths of Ny'alotha to regain its strength. Now it has fused with the city's architecture and spreads across the Black Empire like a creeping infestation. As Il'gynoth's corrupting flesh grows ever larger, so does its power.
This fight has two phases which repeat. After Il'gynoth is defeated his Organs are exposed and you enter Phase 2. Once an Organ has been killed you return to Phase 1 and all other Organs return to full health, however Blood of Ny'alotha now spawn from this location. As the fight goes on more and more Bloods will spawn.
Eye of N'Zoth - This is the tank swap for this encounter. Il'gynoth will cast an eye beam towards the active tank, dealing a large amount of Shadow damage. The tank will then receive a debuff increasing damage taken from Eye of N'Zoth by 75%.
Corruptor's Gaze - A beam will spawn from the roof targeting a player and following them. The path the beam has taken will be covered in 
Morass of Corruption. If any player enters the Morass of Corruption they will take large amounts of Shadow damage, be pacified and silenced.
[Heroic] 
Touch of the Corruptor - Several players will become mind controlled for 1 minute. If these players melee anybody they will also become mind controlled. To break this mind control you need to reduce their health to 30%.
Each of the Organs will require player attention, assign people to interrupt each of the Organs.
Cursed Blood - Throughout the phase several players will receive this debuff. It will deal small amounts of Shadow damage every 2 seconds for 8 seconds and then expire. When the debuff expires it will deal a large amount of Shadow damage to all players within 11 yards.
Pumping Blood - This is the cast that will need to be interrupted. If it is not interrupted a Blood of Ny'alotha will spawn.
Blood of Ny'alotha
Fixate - The Bloods will Fixate on a random target, and move towards them. If they reach their target they will start to melee them. Each melee swing leaves a stack of 
Recurring Nightmare which deals a small amount of damage every 2 seconds for 40 seconds. This debuff has no stack limit.
Hemorrhage - When the Blood dies it will deal a moderate amount of Shadow damage to everybody in the raid.
The Phase ends when one is defeated and the other Organs will return to full health, however Blood of Ny'alotha will spawn periodically from the location of the recently defeated Organ. The next time you enter Phase 2 there will be 1 fewer Organ(s).
Phase 1 and 2
Cursed Blood - This is now a dispellable debuff and is present in both Phases of the encounter. It will grow and shrink in size.
Cursed Pulse - When Cursed Blood is at full size, it will deal a moderate amount of damage to all players within 14 yards. This is indicated by the radius around the character.
Cursebreak - When a Cursed Blood is removed it will deal damage to the raid based on its size. Dispel when it's small.
Note: Removing it via an immunity will still trigger Cursebreak. You are able to pre-immune it.
Phase 2
When you enter Phase 2 a new add will spawn called Clotted Corruption. This add is tankable.
Absorbing Charge - This is a 2 second cast where the add will charge in a straight line and deal large Physical damage to anyone hit. If any Blood of Ny'alotha are in the path it will absorb them gaining 
Reconstituted Blood which grants 10% damage for each Blood absorbed.
LFR/Normal/Heroic
Mythic
Face the boss away from the raid when you are tanking so Eye of N'Zoth does not hit other players
Swap on 2 stacks of Eye of N'Zoth
During the intermission keep the Organs interrupted to ensure additional Bloods don't spawn
[Pending Final Trinket Tuning] Lingering Psychic Shell can be very useful for taking hits from Eye of N'Zoth as when it is activated the entirety of the hit(s) will be mitigated, rather than just the absorbed amount (as of the most recent tests on PTR)
[Heroic] 
Ring of Peace can be useful to keep the players mind controlled with Touch of the Corruptor away
Sigil of Chains can be useful later in the fight to group a lot of Bloods of Ny'alotha together
You can solo an Organ of Corruption during Phase 2 by using 
Avenger's Shield exclusively as an interrupt and fishing for procs
Blessing of Spellwarding can be used to negate/reduce the damage from Eye of N'Zoth however it will not negate the stack of the debuff
[Mythic] Blessing of Spellwarding / 
Divine Shield can be used to negate the debuff from Cursed Blood. However, you will need to cast this before the debuff has been applied.
Rallying Cry can be helpful during Phase 2 as large clumps of players will be taking damage from Cursed Blood
Spell Reflection can be used to reduce the damage from Eye of N'Zoth however it will not reflect the stack of the debuff
[Mythic] Rallying Cry can be helpful during any period of Cursed Blood removal as this triggers raid wide damage
Ursol's Vortex can be useful later in the fight to group a lot of Bloods of Ny'alotha together
You can 
Mass Entangle the Bloods of Ny'alotha to allow them to group up for more efficient cleave
[Heroic] 
Typhoon can be useful to keep the players mind controlled with Touch of the Corruptor away
Gorefriend's Grasp can be useful later in the fight to group a lot of Bloods of Ny'alotha together
Anti-Magic Shell can be used to negate/reduce the damage from Eye of N'Zoth however it will not negate the stack of the debuff
[Mythic] Anti-Magic Shell can be used to negate the debuff from Cursed Blood. However, you will need to cast this before the debuff has been applied.