When she struck her bargain with N'Zoth ten thousand years ago, Queen Azshara offered one of her most devoted followers into the Old God's service. Now Uu'nat and the crawling masses he commands herald the coming of their master, as foretold by the stars.
This is a three phase fight where the boss will gain and lose mechanics each phase along with several adds and your raid will have access to the relics from The Restless Cabal.
Each Phase will have a different Gift of N'Zoth ability active on Uu'nat.
Trident of the Deep Ocean (Tanks)
When used this will create a 
Custody of the Deep shield around the tank which will redirect 75% of damage taken of all players within the shield to the shield itself. Any remaining health on the shield after 20 seconds will trigger 
Abyssal Collapse dealing Frost damage to the raid.
You will want to use this Relic as a raid cooldown and ensure it gets as close to full value as possible to reduce the raid damage afterwards.
This relic will take 2 minutes to recharge, and after using it the tank will recieve the 
Feedback: Ocean debuff for 3 minutes.
Void Stone (Healers)
When used this will place a damage absorbsion shield called 
Embrace of the Void on the Healer, when this expires so do all of the effects of the Void Stone so the healer will need to take damage to remove the effect.
Whilst this shield is active the raid and all enemies will take 100% less healing. This is useful for clearing up 
Oblivion Tears and preventing add healing.
This relic will take 1 minute to recharge, and after using it the healer will recieve the 
Feedback: Void debuff for 2 minutes.
Tempest Caller (DPS)
When used this will create a 
Storm of Annihilation on the player dealing damage to all raid members and adds, if any fall below 25% during this effect they will be killed instantly via 
Annihilate.
During this effect everyone hit will deal 50% less damage, however if any adds die from Annihilate they will not trigger their on death effect.
This relic will take 30 seconds to recharge, and after using it the player will receive the 
Feedback: Storm debuff for 4 minutes.
The main tank swap for this encounter is a frontal cone from Uu'nat called 
Touch of the End - This will force all adds to fixate on the target.
However the Undying Guardian adds have an aura around them called 
Sightless Bond which will make themselves and Uu'nat take 99% less damage if they are within 10 yards of the boss, and a buff on them called 
Shroud of Whispers making them deal more damage the more health they have.
When these adds reach 1 health they will cast 
Unnatural Regeneration and heal back to full health. The only way to kill these adds is with the Tempest Caller relic.
Gift of N'Zoth: Obscurity - When Uu'nat reaches 100 energy he will cast this ability. Once cast the outside of the encounter area will be covered in the Obscruity and if you are inside of this area you will take heavy Shadow damage. This will be cast multiple times and will force you closer into the centre of the room each time.
You will want to start on the edge of the room and work your way in to avoid blocking your path with mechanics on the ground.
Oblivion Tear - This circle will spawn on the location of a random player and should be avoided by players after it spawns. If anyone comes into contact with it they will take shadow damge, be knocked back, and heal Uu'nat for 1% health.
You will want to clear these up with your Void Stone healer where possible (start of Phase 2 is advisable due to the low number of mechanics)
[Phase 1 only] 
Maddening Eyes of N'Zoth - A large part of the room will be covered with this ability and after the 4.5 second cast will take Shadow damage if inside of this and be hostile to allies for 8 seconds.
[Heroic - Phase 1 only] 
Void Crash - Similar to the Void Crash on The Restless Cabal encounter you will want to wait for these to have bounced twice before soaking.
[Normal - Phase 1 only] 
Piercing Gaze of N'Zoth - Two beams will appear from the edge of the room crossing over in the middle. You will need to go through both of these, however when you make contact with one you will receive a Shadow DoT.
Try to move through one as soon as possible and do your second move once the DoT has fallen.
At the start of this phase Uu'nat will move towards N'Zoth and channel for 20 seconds taking 99% less damage. Use this time to kill off any remaining adds.
Gift of N'Zoth: Hysteria - This replaces the previous Gift. When Uu'nat reaches 100 energy it will deal Shadow damage to the raid, heal adds for 50% and give them a 50% haste and movement speed buff.
[Heroic] Adds will be healed for 75% (up from 50%)
[Phase 2 Only] 
Unknowable Terror - When cast a large area of the room will be marked and you will need to move out of it before the cast is complete. If you are hit you will take Shadow damage and be feared for 6 seconds.
New adds for Phase 2
Primordial Mindbender - They will attempt to cast 
Consume Essence which will deal damage and heal them back to full. You can interrupt this cast or use crowd control.
When these die they will spawn multiple Swarming Void Spawns.
Swaming Void Spawn - These adds only melee swing and when they die they will deal Shadow damage to all players within 100 yards via 
Void Eruption. You can negate this on death effect with the Tempest Caller.
[Heroic] Piercing Gaze of N'Zoth returns for this phase.
Gift of N'Zoth: Lunacy - This replaces the previous Gift. When Uu'nat reaches 100 energy it will deal Shadow damage to the raid and make all players become hostile to allies for 5 seconds.
Insatiable Torment - This will make the player unable to recieve healing, and take shadow damage every second. However they will leech health from nearby raid members. Be stacked to negate this mechanic.
Please note: There was no Mythic testing for this boss, so this is from the Dungeon Journal
Coming Soon™
Swap after each Touch of the End cast. It's generally easier to swap positions rather than mobs to avoid the damage reduction effect. You can know in advance when this is going to happen as the debuff will be timing out on the add tank when its about to be cast.
Trident of the Deep Ocean - Only use this at points where you will get full value from the absorb else you may kill your raid.
Void Stone - Utilise defensives here as healing received is reduced by 100%.
Tempest Caller - Stay above 25% health
A pre-placed 
Transcendence can make the tank swap very simple, as well as monks high mobility
Zen Meditation can be used to soak Void Crash in Phase 1 whilst they are large to prevent raid movement from further bounces
Leg Sweep can be used as an AoE Interrupt on the Mindbenders in Phase 2
Ring of Peace can be used as an interrupt on the Mindbenders in Phase 2
Infernal Strike can make the tank swap very simple
Sigil of Chains can be used as an interrupt on the Mindbenders in Phase 2
Sigil of Silence can be used as an interrupt on the Mindbenders in Phase 2
Sigil of Misery can be used as an interrupt on the Mindbenders in Phase 2
Blessing of Spellwarding can see high value in this encounter due to the large amounts of magic based mechanics that go on raid members
Blinding Light can be used as an AoE interrupt on the Mindbenders in Phase 2
Aegis of Light can be used on the raid to lower damage taken from the Tempest Caller to ensure people do not die
Heroic Leap and 
Intercept can make the tank swap very simple
You are able to negate the fear from Unknowable Terror with 
Berserker Rage (however you will still take the large shadow damage hit)
Shockwave can be used as an AoE interrupt on the Mindbenders in Phase 2
Intimidating Shout can be used as an interrupt on the Mindbenders in Phase 2
During Tempest Caller is a good time to use 
Rallying Cry
Stampeding Roar can be useful to help the raid move from numerous mechanics (Maddening Eyes of N'Zoth and Unknowable Terror)
Incapacitating Roar can be used as an AoE interrupt on the Mindbenders in Phase 2
Typhoon can be used to interrupt the Mindbenders
Death's Advance will prevent knockbacks, so you can soak multiple Oblivion Tears with relative ease (if strategy allows)
Be particularly aware of the time your raid will use the Void Stone, as this makes you immune to all healing (including self-healing) while it is up. Consider asking for externals during this time. Life Cocoon should be able to bypass this as it did on Rastakhan.
Anti-Magic Shell can be used to not get a DoT stack from the Maddening Eyes of N'Zoth beam
Anti-Magic Shell can be used to soak Void Crash in Phase 1 whilst they are large to prevent raid movement from further bounces
Grips can be used as an interrupt on the Mindbenders in Phase 2