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.
[Heroic] Unleashed Insanity - All players within 12 yards will be stunned and take a large amount of Shadow damage.
Appendages of Drest'agath
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.
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.
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)
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.
Tank Damage Profile
Swap on each cast of Volatile Seed and put it on your highest priority add(s)
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
Vengeance Demon Hunter
Whilst you are not tanking aim to run around the room and debuff Tentacles with Chaos Brand
Stampeding Roar is very useful to allow the raid to move around the room faster