As he bravely fought to save the Chamber of Heart, Ra-den was pulled into Ny'alotha where he has fallen victim to N'Zoth's whispers. The despair he harbored for so long has overwhelmed the last of his hope, rendering his corruption complete. Now the former Highkeeper will stop at nothing to do the Old God's bidding.
When you pull the boss you will want to re-position him towards the centre of the room as this will allow players to move to the safe spots for 
Burning Cataclysm later in the encounter with more ease.
After each Burning Cataclysm, re-position the boss back to the centre for the next cast. The boss will do 2x Burning Cataclysms, and then transition into Phase 2.
Searing Breath - This is the main tank mechanic of this encounter. This is cast every ~8 seconds, deals damage to everyone in a conal radius in front of the boss and applies the 
Searing Armor debuff which lasts for 20 seconds, therefore you will swap on 2 stacks of this debuff.
Tail Swipe - This ability is only cast if anyone is directly behind Wrathion. Be mindful of player positions when you are conducting tank swaps.
Incineration - The boss will apply this debuff to non-tank members of the raid and the damage it deals to players is based on their distance away from you.
Gale Blast - Wrathion will lift off in the air and create a large zone around him. Anyone inside of this zone will take a large amount of Fire damage and be knocked back. Anyone outside of the zone will take a moderate amount of Fire damage. After the Gale Blast has finished casting, Wrathion will launch some fireballs at players which will have a visual on the floor. Avoid being too clumped together when moving out to allow players to dodge these.
Burning Cataclysm - When this is cast the boss will transform into a fiery cloud and move to one side of the room. After this he will spawn 
Scorching Blisters all over the room with the exception of the direct opposite location. Pay attention to which way the boss moved and start to run in the opposite direction, when you are in the position directly opposite look for the safe area and stand within it.
Tip: You are able to detect where the end location will be before the cast starts as Wrathion will face the direction he will move at the start of the cast. Once he has turned into a smoke cloud you will also be able to see another smoke cloud which indicates the end location.
Once all of the Scorching Blisters have spawned they will expand and explode. Anyone caught within one will likely be dead.
Wrathion will immediately cast 
Molten Eruption and spawn several 
Molten Pools, these will be important in Phase 2.
*Please note the above video was taken during Normal Mode testing, in Heroic + Mythic the safe area will be much smaller.
After 2 Burning Cataclysm's the boss will cast Smoke and Mirrors which begins the next phase.
During this phase each of the Molten Pools left behind by Wrathion during Phase one will transform into a Crackling Shard, during this phase you will be able to destroy these Crackling Shards and remove them from the encounter. Any that are not destroyed will transform into 
Lava Pools which will apply the 
Rising Heat debuff to the raid for each Lava Pool.
Scales of Wrathion - Several pools will appear on the floor, the player soaking it will receive 10 stacks of the 
Burning Madness debuff. This debuff allows you to destroy a Crackling Shard when you run through it, consuming 1 stack for each Shard destroyed and will also extend the duration of the debuff. Players with high mobility should take this.
Any shards that remain should be killed by the raid. Phase one will resume when all shards have been removed.
Heroic - During this phase Ashwalker Assassins will spawn around the room. Shortly after spawning they will go into stealth. Keep an eye on where they spawn and use AoE spells or Flares to take them out of stealth.
If they reach their target whilst in stealth they will stun them for 10 seconds with 
Noxious Choke. You do not want them to stun your players who have the Burning Madness debuff.
Creeping Madness - This is the notable new mechanic for Mythic and will change your approach for Phase 1 to be alot more structured. Creeping Madness will apply a 2.3 minute long debuff on everyone in Phase 1 which times out at the end of Phase 1 and refreshes when Phase 1 begins again.
Each time you move when you have this debuff on you, you will receive a stack of Slow. The slow debuff lasts 45 seconds so you only want to move when absolutely necessary (Burning Cataclysm/Incineration).
At 50 stacks of slow you will take near lethal damage when you move.
Abilities that remove movement impairing effects will reset the stacks of slow on a player so these should be used on players with high stacks that need to move, the ones observed to work are: Shapeshifting (Druid), 
Blessing of Freedom (Paladin), 
Divine Shield (Paladin), 
Tiger's Lust (Monk), 
Cloak of Shadows (Rogue), 
Ice Block (Mage), 
Dispersion (Shadow Priest), 
Vengeful Retreat (Havoc Demon Hunter), 
Master's Call (Hunter), 
Disengage (if talented for 
Posthaste - Hunter), 
Bladestorm (DPS Warrior), 
Avatar (Warrior), 
Demonic Circle: Teleport (Warlock).
Raid Positioning
You will want your raid position to be somewhat strict on Mythic to minimise the total amount of movement required.
You will have the Tanks stand on either side of the boss (but on the same half of the boss so he swings around in front and does not Tail Swipe the Melee DPS) - This is how you lower tank + melee DPS movement.
The Ranged DPS + Healers will stand a small distance away so that they do not need to move but can still heal both Tanks.
When Incineration is cast, this is the only time that players will need to move. These players should ideally have Blessing of Freedom / Tiger's Lust placed on them to reset their stacks. I personally used this weak aura (click here) to assign players to left/right/back to minimise bad movement from players second guessing themselves and ensure an equal spread.
When Gale Blast is cast you will want to have all players run to the ranged stack of players, and then move after the Fireballs have been cast on that location.
After you have moved into a safe location for Burning Cataclysm, place a Warlock Gateway towards the boss and have the entire raid use it to get back to the boss and minimise movement. This is only applicable for the first Burning Cataclysm for each Phase 1 cycle as your debuff will drop for the second.
Hardened Core - This debuff transforms how Phase 2 of the fight functions. As each Crackling Shard will be immune to taking any damage until this has been removed and this can only be removed by Burning Madness. This means that you will always have Lava Pools spawn at the end of Phase 2, adding a soft enrage to the encounter.
At an absolute minimum you will have 2 Lava Pools.
Position the boss so he does not Tail Swipe any players.
Swap on 2 stacks of Searing Armor.
Always re-position the boss to the centre to allow for easier raid movement.
[Mythic - UNTESTED] 
Null Barrier major can remove stacks of Slow on yourself.
The speed boost from 
Provoke is useful to re-position the boss after Burning Cataclysm.
[Mythic] Tiger's Lust can be used to remove Slow on players
Fiery Brand can be used to reduce the damage taken from the second stack of Searing Armor
[Heroic] 
Sigil of Misery and 
Sigil of Flame can be used to break the Assassins out of stealth
Blessing of Spellwarding and Divine Shield can be used to negate a stack of Searing Armor (Please note: You will need to use this ability when Wrathion has started to cast Searing Breath, else he will target another player)
[Mythic] Blessing of Freedom can be used to remove Slow on players
[Mythic] Divine Shield can be used to remove Slow on yourself
Spell Reflection should be used to reduce the damage taken from the second stack of Searing Armor
[Mythic] Avatar can be used to remove Slow on yourself
Utilise 
Stampeding Roar to help your raid move around more effectively
[Mythic] Shapeshift can be used to remove Slow on yourself
Anti-Magic Shell can be used to reduce the damage taken from the second stack of Searing Armor
[Heroic] You are able to grip the Assassins together during Phase 2
[Heroic] 
Death and Decay can be used to break the Assassins out of Stealth
[Mythic] Anti-Magic Shell can be negate stacks of Slow building on yourself for its duration.