Phase 1

This phase is a case of defeating the many adds that spawn while healing Kael'thas Sunstrider.

There are 6 adds with mechanics that spawn in fixed positions. These are:

High Torturer Darithos is present for the pull, and periodically casts greater castigation dealing moderate shadow damage to random selected players and anyone within 6 yards every 0.5 seconds over 6 seconds
Vile Occultists will also be present for the pull. They periodically teleport across the room, as well as casting the interruptable vulgar brand that applies a debuff to all raid members. also drop essence font, which can be healed, which in turn heals Kael'thas Sunstrider
Bleakwing Assassin who will mark a player and then use crimson flurry that applies a large DOT to all within 8 yards. At low health they channel return to stone, an 8 second cast that completely heals them at completion
Rockbound Vanquisher who stacks vanquished on the tank. They also periodically cast concussive smash that causes damage and increases raid damage by 25%.
Pestering Fiends that spam shattering ruby. When killed they cast fragmentation.
Soul Infuser that slowly move to Kael'thas Sunstrider and cast soul infusion. When killed they drop infusers orb which will give a player the infusers boon.
There will be points at which there are no adds. All players who can, should dump all possible healing into Kael'thas Sunstrider

Tanks also be affected by:

Other mechanics of note:

Healers can heal Kael'thas Sunstrider.
soul pedestal can be used to channel a player's health to Kael'thas Sunstrider. This will apply a debuff to the player.

Approach throughout phase 1:

Adds should be dealt with in a priority order
The soul pedestal should be used in a controlled fasion
High Torturer Darithos Should be tanked on pull, with players spreading when greater castigation is cast. Due to his low damage output, he is a low priority target and should be cleaved when possible.
Vile Occultist should be interrupted and brought down as soon as is feasible to stop this ongoing interrupt requirement
Bleakwing Assassin adds need to be prioritised when up, but particularly need to be burst down before return to stone completes
Rockbound Vanquisher should be killed as quickly as convenient to prevent the stacking casts of vanquished. This should be the main padding target of a cleave rotation.
Pestering Fiends die quickly, but their fragmentation causes raid wide damage that can stack in a nasty way. The ranged should attempt to cleave these as the raid moves and kites them around the room, ensuring that they are not near players when they die. Melee should quickly move away if these adds are near death.
Soul Infuser adds are the main priority and must be burst down quickly, before they cast soul infusion. A healers should then pick up the infusers orb and use it to heal Kael'thas Sunstrider.

Tanks should

Healers should:

Ensure that the raid is topped off, paying attention to concussive smash.
When the raid is stable, healing should be focussed on Kael'thas Sunstrider
Players will rotate using soul pedestal and these should also be topped off as they will lose health
When an infusers orb is dropped, these should be picked up and Kael'thas Sunstrider should be healed as a priority to make the most of the buff
A healer with a the healing buff should then pick up the next orb as this effect stacks and refreshes and ensures an exceptionally high healing throughput for that player

Key mechanics to master:

Killing adds in the correct order
Moving out the groups with debuffs such as greater castigation as well as moving away from adds with effects such as fragmentation

Phase 2

Phase 2 occurs at 40% & 90% of Kael'thas Sunstrider remaining health. In this phase, Shade of Kael'thas will appear, as well as reborn phoenix adds.

At the start of the phase:

Abilities:

fiery strike, a frontal cone causing burning remnants. He also selects a player and casts a second cone effect: blazing-surge which is avoidable if they are quick to move. This also leaves fire on the floor.
ember blast is periodically cast on a random raid member, dropping more fire on the floor and applying a DOT.

During the phase:

reborn phoenix will spawn, pulsing smoldering plumage and dropping smoldering remnants underneath them
reborn phoenix will respawn after 20 seconds of being dead

Tanks should

Ensure that the stacks of burning remnants are managed by swapping once the effect has dropped off

Healers should:

Ensure that the players affected by ember blast are quickly topped back up, and that the subsequent DOT is also healed

To complete this phase:

The raid should follow the Shade of Kael'thas while the tanks kite him around the outside of the room
The raid must avoid all fire effects on the floor
Players with ember blast should move out of the raid, 10 yard behind the group
Approximately a third of the raid should help soak players with ember blast but try not to get it twice as the DOT can be nasty
blazing-surge can be quickly avoided by sidestepping. As it is a cone, being nearer to the boss makes this easier.
Ranged players should slow the reborn phoenix adds and kill them when they are alive

This phase will end when the boss has lost 10% of its health