The grotesque amalgam of flesh known as Drest'agath slumbered beneath Ny'alotha for millennia until being awakened by N'Zoth's return. Now she rises like a revolting boil, ready to burst and spread her vile pestilence across Azeroth.
This is a single phase encounter where the boss will heal all of the damage you do to him unless you have the buff from 
Void Infused Ichor which drops from the adds during the encounter.
Void Grip - If nobody is in range of the boss he will grip his target and stun them for 5 seconds.
Volatile Seed - This will act as the tank swap. The active tank will have the Volatile Seed placed on them and after 10 seconds it will detonate. Any friendly or hostile targets within 13 yards will be inflicted with 
Volatile Corruption. This includes the tank that placed it.
Volatile Corruption - This will deal moderate Shadow damage to all targets and increase damage taken by 60% (this also includes enemies).
Aberrant Regeneration - The boss will heal all damage dealt to it, unless the player is buffed with Void Infused Ichor.
Entropic Crash - All players will take damage based on their distance from Drest'agath. If any Tentacles of Drest'agath are alive they will also cast Entropic Crash.
Void Glare - A beam will be cast from the boss towards a player location. If any Eyes of Drest'agath are alive they will also cast Void Glare.
Throes of Agony - When Drest'agath reaches 100 Agony he will channel this ability dealing moderate amounts of Nature damage every 2 seconds for 10 seconds.
[Heroic] 
Mutterings of Insanity - One random non-tank member of the raid will be slowed over 5 seconds, at the end of the 5 seconds they will trigger 
Unleashed Insanity. If any Maw of Drest'agath are alive they will also cast Mutterings of Insanity.
[Heroic] Unleashed Insanity - All players within 12 yards will be stunned and take a large amount of Shadow damage.
There are three types of adds which all have unique abilities and ways to be handled.
When any of these die they will drop Void Infused Ichors. These allow you to deal damage to Drest'agath for 30 seconds, however after the buff expires you will receive the 
Umbral Aversion debuff which makes you unable to collect Void Infused Ichors for 1.5 minutes.
Note: The Umbral Aversion debuff was not present in any of my testing.
Dismemberment - When any add dies they transfer 5 Agony every second to Drest'agath for 5 seconds.
Eye of Drest'agath
Mind Flay - Channels low amounts of Shadow damage on a target, and slows their movement speed for 5 seconds.
If Drest'agath is at 100 energy and the Eye of Drest'agath is alive it will cast 
Errant Blast, this will make the Eye blast the roof and create a Red zone on the floor. If you are inside of this you will take damage from the 
Falling Gore.
Maw of Drest'agath
Mutterings of Betrayal - This will apply a healing absorb to a targeted player in the raid and all players within 10 yards. This debuff can stack. If the debuff ever stacks to 4 the player will become hostile and get the 
Betrayed debuff.
Note: On PTR this seemed to affect pets.
Acid Splash - This will target a random target and throw acid at them. Anyone within 4 yards of that player will take a moderate amount of Nature damage.
If Drest'agath is at 100 energy and the Maw of Drest'agath is alive it will cast 
Spine Eruption, this will make the Maw shoot out rings of spines around themselves. If you are caught in any of these you will take a large amount of Physical damage every 3 seconds for 9 seconds.
Tentacle of Drest'agath
Note: On PTR the Tentacle would not cast any abilities if nobody was in melee range.
Crushing Slam - If a player is in melee range of the Tentacle it will highlight an area on the ground with a small red swirl and then slam shortly after. If anyone is caught in this they will take a large amount of Shadow damage.
Void Miasma - A cloud of Miasma will be around the Tentacle inflicting a moderate amount of Shadow damage to anyone within it and the players in the Void Miasma are unable to target in or out of the Void Miasma. In order to deal damage to the Tentacle, or heal players inside you will need to be inside of the Void Miasma.
Note: Targeted AoE spells can be cast inside of the Void Miasma and still damage the tentacle (Blizzard for example)
If Drest'agath is at 100 energy and the Tentacle of Drest'agath is alive it will cast 
Reality Tear, this will create a slow moving projectile which will shoot from the Tentacle to the other side of the room. If you are caught within it, you will take a large amount of Shadow damage.
Throes of Dismemberment - This is the new mechanic for Mythic which will trigger the Errant Blast, Reality Tear or Spine Eruption from all remaining adds of that type when they are killed.
Swap on each cast of Volatile Seed and put it on your highest priority add(s)
Don't tank after placing a Volatile Seed as you will have the Volatile Vulnerability debuff
Ensure a tank is always in range of Drest'agath
Your high mobility is useful for placing Volatile Seed on far away Tentacles
Whilst you are not tanking, aim to run around the room and debuff Tentacles with 
Mystic Touch
Crackling Jade Lightning can be used to apply Mystic Touch to the Tentacles
Infernal Strike is useful for placing Volatile Seed on far away Tentacles
Whilst you are not tanking aim to run around the room and debuff Tentacles with 
Chaos Brand
In order to reduce the damage from the Volatile Seed explosion, 
Fiery Brand will need to be placed on Drest'agath as he is the source of the damage
Sigil of Flame can be used to apply Chaos Brand to the Tentacles
Divine Steed should be used to place Volatile Seed on far away Tentacles
Blessing of Spellwarding will negate the damage from the Volatile Seed exploding however it will not prevent the Volatile Vulnerability debuff
Heroic Leap is useful for placing Volatile Seed on far away Tentacles
Rallying Cry will be helpful to use during Entropic Crash if multiple Tentacles are up
Spell Reflect can be used to deflect the Volatile Seed debuff and make it not be cast on you. You should only do this when there are no appendages alive to debuff.
Stampeding Roar is very useful to allow the raid to move around the room faster
Wraith Walk will need to be talented for placing Volatile Seed on far away Tentacles
Anti-Magic Shell will reduce/negate the damage from the Volatile Seed exploding however it will not prevent the Volatile Vulnerability debuff