The harbinger for the end of days, Za'qul shreds the last vestiges of sanity from a world in chaos, laying the foundation of Azeroth's new master.
There are three short phases which will introduce you to the mechanics of each realm and a final phase where you will have to manage them all at the same time.
When you engage the boss get onto the boss platform to avoid being hit by the 
Dark Beyond (as denoted by the purple circle on the map above)
On the pull the boss will cast 
Mind Tether on the tank and the closest player to the tank. For this reason you want to ensure both tanks are together on the pull.
Phase 1 (100 - 85%)
During this phase you are in the Normal Realm and will be introduced to the main mechanics of the encounter.
Mind Tether - 50% of the damage taken from the main target will be redirected to the linked player as Shadow damage. This damage increases if they are more than 12 yards away from each other. This ability is cast at the start of each phase and if there is no player within 12 yards of the tank when its cast the tank will be stunned for 10 seconds via 
Snapped.
Crushing Grasp - A portal will be opened up against the edge of the encounter area and shortly after a tentacle will spawn and hit everything in a line directly to the other side of the room from where it spawned. If you are hit you will be knocked back and take heavy Shadow damage.
Dread - This debuff will be cast on random DPS players and they will be feared for 10 seconds. When this fear is removed (Tremor, Dispel, Timing out, Immunity etc) it will deal a large burst of Shadow damage to the entire raid. This will be the cause of the majority of the raid damage of the fight.
Portal of Madness - 3 Shadow zones will appear around the room in a triangle formation. If you are inside of these when they explode you will be feared and take a large burst of Shadow damage. After they explode a Horrific Summoner add will spawn and they will channel Dark Tear which will spawn Horrific Vision adds. If the channel completes the raid will be one shot and you will wipe.
[Heroic] The Horrific Vision adds that spawn will have 30 stacks of 
Gathering Nightmare. For each stack they have they will take 5% more damage and move 2% slower, however they will lose 1 stack every second. When all stacks have been removed they will transform into an Unleashed Nightmare add.
[Heroic] If the Horrific Vision adds melee you, you will have a stack of 
Hysteria applied via its 
Quivering Claws passive.
When the boss reaches 85% Phase 2 will begin.
Phase 2 (85 - 70%)
During this phase you will be dragged into the 
Fear Realm. The raid will receive a stack of Hysteria every 15 seconds whilst they are in this realm.
Maddening Eruption - A large Shadow zone will spawn typically on the opposite side of the room from where the boss is. After 20 seconds it will explode dealing large Shadow damage to the entire raid.
However, if Za'qul is placed in this Shadow zone he will receive the 
Punctured Darkness debuff making him take 30% more damage for 20 seconds.
Manifest Nightmares - This will be placed on the two players furthest from the boss and for 6 seconds they will deal Shadow damage to everyone within 7 yards of them. For each tick of damage a 
Nightmare Pool will spawn. When the debuff expires a Horrific Vision add will spawn.
When the boss reaches 70% Phase 3 will begin.
Phase 3 (70 - 50%)
During this phase you will have access to the 
Delirium Realm.
Delirium's Descent - When this is cast Za'qul will spawn 3 zones which need to be soaked by 1 DPS player each (preferably players with cooldowns available). These 3 players will be sent into the Delirium Realm.
Whilst you are in this realm you will gain 80% haste, and gain 1 stack of Hysteria every 6 seconds but everyone will be hostile to you.
Inside the Delirium Realm you will be able attack the boss, and the adds. You will also take damage from the Crushing Grasps and Nightmare Pools.
[Heroic] Tentacles will spawn and slam down on the ground in projected areas. Avoid these to not take damage, and be stunned from 
Tentacle Slam.
When you die in the Delirium Realm in Phase 3 you will be returned to the Fear Realm and be stunned for 6 seconds via 
Shattered Psyche.
When the boss reaches 50%, Thalyssra will appear and Phase 4 will begin.
Phase 4 (50 - 0%)
You will be dragged back to the Normal Realm and a 
Reality Portal will appear. When you are in any other realm during the encounter to get back to the Normal Realm move near this portal and use the new extra action button.
Shortly into the energy cycle, Za'qul will cast Dark Passage which will open a portal to the Fear Realm and spawn 3 Horrific Summoners. These can only be damage in the Fear Realm.
When the boss reaches 100 energy, he will apply a 5% health shield to himself and start to cast Dark Pulse. If this 18 second cast completes heavy Shadow damage will be taken by every player in the raid.
The Dread ability is now 
Manic Dread - After this has been dispelled a pool of Caustic Delirium will spawn. This will deal increasing damage to anyone that stands within it, and after 5 seconds that player will be transported to the Delirium Realm.
Mythic
3 Crushing Grasp tentacles will spawn each time and trigger in a staggered way.
Instead of Dark Pulse, the boss will cast 
Psychotic Split - An add will spawn in the Fear and Delirium Realms which needs to be killed to interupt the cast and avoid the wipe.
Echo of Fear - This is the add that will spawn in the Fear Realm. It will cast Dread Scram. Keep this cast interrupted.
Echo of Delirium - This is the add that will spawn in the Delirium Realm. It will cast 
Void Slam. Avoid the affected area.
Boss Tank:
Linked Tank:
Stay near your co-tank to keep Mind Tether damage to a minimum
Be observant of Crushing Grasp spawn locations and move the boss as necessary
In the Fear Realm (Phase 2 + 4) move the boss into Maddening Eruption.
During Phase 4 stay in the Normal Realm and have a player in the Fear Realm mark where the Maddening Eruption has spawned.
Use your taunts on the adds that spawn throughout the encounter, as you will not need it for the boss it is safe to do so.
The speed boost from 
Provoke is useful to move the boss into Maddening Eruption in the Fear Realm
Black Ox Statue can be useful to group adds together and taunt them into the boss
As a Brewmaster you should be the main tank on the boss to lower the Linked damage
You can move the boss quickly to the Maddening Eruption in the Fear Realm by utilising both 
Infernal Strike charges
If you are the boss tank, use 
Blessed Hammer. If you are the add tank, use 
Holy Shield.
Using 
Blessing of Sacrifice on the boss tank will reduce the damage taken by both tanks.
If you are the Linked tank, utilise 
Ignore Pain and 
Spell Reflect to lower your damage taken
Using 
Intervene on the boss tank will reduce your damage taken if you are the Linked tank.
Utilise 
Stampeding Roar to help your raid move around more effectively
You can grip the adds into the boss for easier cleave
If you are about to get hit by Crushing Grasp, Death's Advance will prevent the knockback
As a Blood Death Knight you should avoid being the main tank on the boss as the Linked damage would be high