From his titan-forged prison deep beneath the waves, N'Zoth spent eons subtly spreading his influence across Azeroth. Now free from captivity, the Corruptor reveals his true form as he enacts his ultimate plan to make his vision of the Black Empire our new reality. If he is not stopped, all we have ever known will be remade in the image of his eternal madness.
This is a three phase encounter where you will alternate between inside of N'Zoths mind and outside of it. During each phase you must deal with various adds in order to reach the final phase where you can attack N'Zoth directly.
Throughout the entire fight you will have a 
Sanity bar, this functions in the same way that it did for the Carapace of N'Zoth encounter where if your Sanity ever reaches 0 you will gain the 
Gift of N'Zoth buff which will provide you with a 100% increase to your damage and healing done for 20 seconds. However, when this buff expires you will become a 
Servant of N'Zoth, and repeatedly cast 
Mental Decay on players in the raid which will reduce their Sanity.
Please note: During testing you were able to remove the Servant of N'Zoth mind control effect with 
Tremor Totem. I would highly suspect this to be fixed for when it goes live.
You are able to restore your Sanity with your extra action button ability 
Azeroth's Radiance which will start a 4 second channel restoring Sanity every second.
If you move or cast a spell the channel will stop.
As soon as the encounter begins N'Zoth will transport everyone into the Mindscape where you will face against Psychus, and the Exposed Synapse tentacles. N'Zoth will have a shield around himself reducing his damage taken.
Mind's Eye
These surround the encounter area and will open periodically. When they do open they will cast Void Gaze.
Void Gaze - This ability will deal a small amount of Shadow damage and drain 5% Sanity from all players every second for 3 seconds.
Exposed Synapses
Probe Mind - Deals moderate Shadow damage to random players within 30 yards. If any players are within 5 yards of the target it will chain to them.
Resilient Synapses - When an Exposed Synapse is killed, all others will heal to full.
Psychus
Synaptic Shock - If Psychus is within 8 yards of an Exposed Synapse when it dies he will gain this buff which increases his damage taken by 50% per stack for 30 seconds.
Mindwrack - This ability will deal moderate amounts of Physical damage to the tank and apply a debuff which increases Physical damage taken by 33% for 14 seconds.
Creeping Anguish - Psychus will stand still to cast this ability, once it has cast an Anguish zone will appear under him and it will grow to 60 yards in size. If any players are stood within it, they will be affected by Anguish. They will take large amounts of Shadow damage, lose 10 Sanity every 2 seconds and their movement speed will be slowed by 33%.
When Psychus has been killed you are able to interact with your body that was created when you entered the Mindscape and 
Restore Consciousness to return to the Physical Realm.
N'Zoth and all tentacles alive will also be stunned for 30 seconds from 
Shattered Ego, this stun removes the damage reduction shield temporarily on N'Zoth in the Physical Realm.
If you remain in the Mindscape for too long you will be hit by the 
Collapsing Mindscape and die.
This Phase ends when you leave the Mindscape.
During this phase you will be in the Physical Realm and N'Zoth will defend himself with his tentacles. You will not be able to attack N'Zoth directly as he will continue to take reduced damage.
N'Zoth
Mindgrasp - The inside or the outside of the encounter area will be chosen and shortly after you will be dragged towards that location.
Corrupted Neuron - These will spawn all over the room, regenerate 1% health every 2 seconds, and project a Veil of Anguish from their locations, the centre of the room, and the perimeter. The size of the Veil of Anguish around the Corrupted Neuron will reduce as their health is lower. If any players are stood within the Veil of Anguish, they will be affected by 
Anguish. They will take large amounts of Shadow damage, lose 10 Sanity every 2 seconds and their movement speed will be slowed by 33%.
Paranoia - Pairs of players will be chosen and have this debuff applied to them for 15 seconds. Players with the debuff need to be within 5 yards of their partner else they will take light Shadow damage every second which then increases over the duration of the Paranoia.
Whilst you are under the effects of Paranoia all non-partner players will make you lose 5 Sanity every second if you are within 5 yards of them.
Eternal Torment - All players in the Physical Realm will take a light amount of Shadow damage when this is cast.
Mindgate - A portal to the Mindscape will open and half of the raid can enter at a time.
All Tentacles
Corrupted Viscera - When slain the Tentacle will explode. Corrupted Viscera will be scattered and it will deal a large amount of Shadow damage, and drain 5 Sanity from all players struck by the debris.
Basher Tentacle
Tumultuous Burst - If no players are in range of the Basher Tentacle it will cast this spell. This will deal a moderate amount of Shadow damage to all players every 2 seconds for 10 seconds, and reduce Haste by 33% for 10 seconds.
Void Lash - This will deal a large amount of Physical damage and a moderate amount of Shadow damage every 0.8 seconds for 6 seconds. You will need to use a defensive/external on this.
Evolving Might - The tentacle will gain a stack of this buff every melee swing they make. This buff will increase their damage by 3%.
Corruptor Tentacle
Corrupted Mind - This spell can be interrupted. If it is not interrupted it will apply a debuff on the target which will deal a small amount of Shadow damage every 2 seconds for 10 seconds. If this debuff is not dispelled the player will lose 15 Sanity.
Spike Tentacle
This is the lowest priority to kill and for the most part can just be multi-dotted.
Pain Spike - This will deal a light amount of Shadow damage on their target.
Mindgate
This Phase is the same as Phase 1 however either Deathwing or Azshara & Sylvanas will join.
You will ideally have a set path in mind for how you handle this phase. Linked is an example of mine.
You will want to bait the Creeping Anguish at set locations to not cut your path.
Deathwing
Cataclysmic Flames - Fire circles will appear on the floor and you will need to avoid these. All players inside the Mindscape will take a small amount of Fire damage and have 5 Sanity drained every 3 seconds for 9 seconds. If you are hit by any of these Flames you will gain the 
Flames of Insanity debuff.
Flames of Insanity - You will take a moderate amount of Fire damage and lose 20 Sanity instantly. You will be burning and take a small amount of Fire damage every 2 seconds for 6 seconds and be disoriented for the duration.
Azshara & Sylvanas
Please note: During my PTR testing this Phase did not trigger.
Tread Lightly - If you move, you will trigger 
Contempt. Contempt deals a light amount of Frost damage every 2 seconds until you come to a complete stop.
Black Volley - All players in the Mindscape will take a small amount of Shadow damage and have 5 Sanity drained every second for 3 seconds.
Phase 3 will begin once the whole raid has entered and left the Mindscape.
During this phase you are able to attack N'Zoth directly and he will retain his Mindgrasp and Paranoia abilities. He will also spawn Thought Harvester's throughout the Phase which need to be dealt with as a high priority.
N'Zoth
Evoke Anguish - This debuff will be applied to several players and lasts for 8 seconds. The players will take a moderate amount of Shadow damage every 5.2 seconds and when the debuff expires an Anguish puddle will spawn on the floor. This puddle will grow in size after it has spawned and each subsequent cast of Evoke Anguish will make existing Anguish puddles grow in size.
This needs to be managed well else you will run out of space.
Stupefying Glare - Eyes will spawn at N'Zoth based on player locations. After a few seconds they will start to channel void energy for 4 seconds. Any players caught in these will take a moderate amount of Shadow damage and be feared for 4 seconds.
Thought Harvester
Mindwrack - This ability will deal moderate amounts of Physical damage to the tank and apply a debuff which increases Physical damage taken by 33% for 14 seconds.
Harvest Thoughts - The Thought Harvester will stand still and cast this for 5 seconds. It will create a 15 yard ring around itself and at the end of the cast any players within it will take a large amount of Shadow damage and lose 100 Sanity divided by the amount of players stood within it (Example: If 10 players are in, they will lose 10 Sanity each).
If this ability does not hit at least 5 players, the entire raid will lose 50 Sanity.
Phase 1 from Normal and Heroic has been removed. You will begin the fight in Phase 2.
Exposed Synapse
When an Exposed Synapse is killed a Psychopage add will spawn.
Psychopage
Devour Thoughts - This will deal a large amount of Shadow damage and drain 10 Sanity from all players within 10 yards.
Psychic Burst - When a Psychopage is killed it will deal a large amount of Shadow damage and drain 30 Sanity from all players within 5 yards.
Psychus
When you kill Psychus you will only stun N'Zoth and not the tentacles around the room.
There is an additional Phase with no details other than Magni and MOTHER joining the battle.
Phase 1 - You can swap after each cast of Synaptic Shock
Phase 2 - 1 Tank should be in each Mindscape Group
Phase 2 - The tank that is in the Physical Realm needs to ensure they are on each Basher Tentacle when it spawns
Phase 2 - Utilise defensive cooldowns or externals for each Void Lash cast
Phase 3 - Swap on each cast of Mindwrack
Your high mobility is useful in Phase 2 to get to new Tentacle spawn locations to prevent the Basher from being alone
Infernal Strike is useful in Phase 2 to get to new Tentacle spawn locations to prevent the Basher from being alone
Mindwrack and the Physical portion of Void Lash are blockable
Divine Steed should be used to move around the room to get to new Tentacle spawn locations in Phase 2 to prevent the Basher from being alone
Mindwrack and the Physical portion of Void Lash are blockable
You can Deflect Anguish, Void Gaze and Paranoia with 
Spell Reflection
You can Reflect Probe Mind and the magic portion of Void Lash with Spell Reflection
Heroic Leap is useful in Phase 2 to get to new Tentacle spawn locations to prevent the Basher from being alone
Stampeding Roar can be very helpful with moving around the room in Phase 2
Wraith Walk should be talented and used to move around the room to get to new Tentacle spawn locations in Phase 2 to prevent the Basher from being alone
[UNTESTED]
Anti-Magic Shell can be used on Void Gaze to prevent the damage and Sanity drain (?)