This is a three phase encounter where the boss will gain a new ability in each phase. The core mechanics of the boss will remain the same in each phase, but how you handle each of the mechanics of each phase changes how you tackle the encounter.
[Beta observation] Each phase would have an independent set of timers. This means that you are unlikely to have bad overlaps when you push into a new phase as the timers completely reset.
Throughout the fight the boss will use the following abilities:
Dimension Tear - This is the main tool at your disposal for dealing with the mechanics of each phase of the encounter. Two non-tank players will be chosen and receive an 8 second debuff. After 8 seconds they will place either end of a portal at their location for you to use. If you run into one of these, you will appear at the other. These portals have a 3 second cooldown which is represented by a debuff.
You will want one player to be close to the raid, and one player at the designated end point of the portal.
Glyph of Destruction - This is the primary tank mechanic of the fight. The active tank will receive this debuff and start to take moderate ticking Arcane damage every second. After 4 seconds the Glyph will explode dealing raid wide damage. The amount of damage dealt is reduced with the distance you are from the other players. After your Glyph explodes you will also receive an Arcane Vulnerability debuff which increases your Arcane damage taken by 50% for 45 seconds.
Rift Blast - Several rifts will appear next to players and project a line in the direction of another player. After a few seconds they will fire a blast of energy which will deal a large amount of Arcane damage. Simply dodge the beams when they spawn.
Hyperlight Spark - This spark will bounce between players in the raid and deal a moderate amount of Arcane damage each time it bounces to that players. It's simply unavoidable damage throughout the encounter.
[Heroic] Stasis Trap - These traps will spawn at the locations of several players periodically throughout the encounter. After 4 seconds they will be armed and if you run into them you will be stunned for 2 seconds, take a moderate amount of Arcane damage every 2 seconds and the trap will become attackable. The trap will need to be killed for you to be released from the trap.
Ideally traps should be placed in low traffic areas (not near the portals) and can be cleared by people with immunities.
Phase 1 (Crystal of Phantasms: 100% - 70%)
The core mechanics of this phase come from the Crystal of Phantasms which will spawn every ~40 seconds and has two mechanics.
Soul Singe - When the crystal spawns it will deal a low amount of Shadow damage to random raid members.
Fleeting Spirit - Several players will have a green fixated marker above their head. This is the indicator that they will have one of these Fleeting Spirit adds chasing them. They will chase for 12 seconds and then despawn. If the spirit makes contact with the player they will become mind controlled via Posession and need to be reduced to 50% HP to end the mind control.
Make use of the portals to kite the spirit. If you stand near one when you first get fixated, wait for the ghost to get close and then walk into the portal. Move out of the portal slightly (as if you stay in it, you will get ported again once the debuff is over), and wait for the spirit to come close again. Port back to the group and the fixate should be ending shortly (if portals are far enough away from each other).
Phase 2 (Root of Extinction: 70% - 40%)
The core mechanics of this phase come from the Root of Extinction which will spawn every ~45 seconds and has two mechanics.
Withering Touch - A random (unlucky) player will receive this 21 second long debuff which will deal a large amount of Nature damage every 1.5 seconds. This player will need to be spot healed. This debuff will be applied before the Seeds of Extinction spawn, so if the player with Withering Touch was assigned to this task, someone else will need to do this.
Seed of Extinction - 4 seeds will spawn in the room. One North, South, East, and West. After 20 seconds they will explode dealing a lethal amount of Nature damage to players within 50 yards of each seed. You can however, pick up these seeds and move them.
Whilst the spell data/dungeon journal says 8 seeds, in every test this was in fact 4 seeds.
You will want to assign 3 players (and backups) to pick up seeds from your current location and the two closest locations and move them all onto the remaining seed. The seeds all have pre-set locations so you can stand in position ready to move them when they spawn.
Phase 3 (Edge of Annihilation: 40% - 0%)
The core mechanics of this phase are from the Edge of Annihilation which will spawn every ~40 seconds.
Aura of Dread - Whilst the relic is present there will be raid wide ticking Shadow damage.
Edge of Annihilation - The relic will activate and everybody in the raid will be drawn towards it at an increasing speed. After 10 seconds, anyone who is within 25 yards of it will take a lethal amount of Shadow damage.
Make use of movement speed buffs, warlock gateway, and adjusted portal placements to counteract this mechanic.
The main change for Mythic is that, the relics persist throughout every phase. This means that in Phase 2 you will have Crystal of Phantasms as well, and in Phase 3 you will have Crystal of Phantasms and Seed of Extinction as well!
The mind control from Phase 1 (Possession) can no longer be broken, the player will instead be Mind Controlled and have to be killed if they make contact with a spirit.
The Seed of Extinction from Phase 2 now 8 seeds (up from 4).
On later tests on Mythic this was triggering the expected 8, however it also said 8 on other difficulties and spawned 4. Will have to wait and see what the live encounter is like!
This section will be updated once a strategy has been devised for Mythic on live servers
Tank Damage Profile
Swing Timer: 1.5s (Curse of Weakness 1.8s)
You should have Celestial Brew available for each Glyph of Destruction to prevent you from being at low health after the explosion. Should Celestial Brew not be available, Dampen Harm and Zen Meditation are both strong damage reductions for this.
Vengeance Demon Hunter
Intervene can be used on your co-tank to mitigate some of their damage taken whilst you are not tanking.
Rallying Cry will likely be used during periods of high raid damage in Phase 2/3.