The dark pact Sylvanas struck with the val'kyr forged her bond with the Jailer. Through the years, these winged sisters faithfully served the Dark Lady, even if it meant sacrificing themselves on her behalf. Now Sylvanas calls her nine val'kyr to fight for her one last time.
Swap on 3 stacks of 
Unending Strike (Phase 1) or 
Pierce Soul (Phase 2).
Make sure the Formless Mass does not complete any casts of 
Siphon Vitality.
Interrupt 
Song of Dissolution on Signe ASAP.
When Kyra is at 100 energy run out of her circle.
When Signe is at 100 energy run into her circle.
[Heroic] 
Word of Recall repeats the last Call of the Val'kyr.
Your speed bonus from 
Provoke is very useful to move the boss(es) out of the damage reduction from 
Annhyldes Bright Aegis.
Sigil of Silence can be used to silence the Formless Mass adds and bring them into melee range.
Pierce Soul and Unending Strike stacks can be removed with 
Blessing of Protection or 
Divine Shield.
Blessing of Spellwarding can be placed on another player to allow them to solo soak 
Arthura's Crushing Gaze.
Soulful Blast can be reflected with 
Spell Reflection.
Stampeding Roar is useful here to allow players to move out with 
Daschla's Mighty Impact or to get to their soaks for 
Aradne's Falling Strike.
Grip in Formless Mass adds when they spawn so they can be cleaved down. If 
Mind Freeze is on cooldown, you can stop their cast with 
Asphyxiate.
During this fight you will fight against 3 Valkyr (even though the name of the fight indicates there are 9 to fight against). There are 6 Valkyr who will fight when called on with 
Call of the Val'kyr. The fight will focus on your ability to play against a wide variety of mechanics which should all be familiar to you if you have raided in the last few raid tiers.
In this phase you will fight against Kyra & Signe with Skyja flying above lending support.
Kyra
Unending Strike – This is the primary tank swap on the encounter. Each hit will leave a debuff that increases damage taken from Unending Strike by 10% per stack. In addition, Repeating Slash will be cast, which will hit all players in the raid.Note: It is possible to Dodge/Parry the Repeating Strike.
Wings of Rage – When Kyra reaches 100 energy, she will pull players towards her for 7 seconds. Any players within 10 yards will be stunned and take a large amount of Physical damage.Whilst Kyra is alive she will summon Formless Mass adds which will require additional focus.
Siphon Vitality – The Formless Mass will attempt to cast this spell, it must be interrupted. If the cast is successful it will deal a moderate amount of raid wide damage and heal for the amount of damage it inflicted. Soulful Blast – This is the main cast from Signe and can be interrupted. Any casts that are not interrupted will deal a moderate amount of Shadow damage to the tank.Song of Dissolution – Every ~20 seconds Signe will channel this spell. It will apply a stacking DoT to the raid every 1.5 seconds. Interrupt this as soon as possible.
Reverberating Refrain – When Signe reaches 100 energy, she will push players away from her for 7 seconds. Any players not within 10 yards will be feared and take a large amount of Shadow damage.Skyja
This boss will not be active during Phase 1 however, when she reaches 100 energy Call of the Val'kyr will be cast. This will summon two Val'kyr to join the fight. Details of what each of the Valkyr can do is detailed in the section below.
Phase 2 will begin when either Kyra or Signe reach 5% (LFR) / 10% (Normal) / 15% (Heroic) health.
At the start of this phase, you will have all 3 bosses active. You should prioritise finishing Signe to remove the necessity for interrupts and then Kyra.
Skyja
Pierce Soul - This is the tank swap of the phase. Each stack will reduce healing received (and movement speed) by 10%.
Resentment - Passive raid damage. Low amounts of Shadow damage pulsing throughout the phase.
Fragments of Destiny- 3 debuffs will go onto players in the raid. When dispelled, it will jump to the nearest player. Once all debuffs are on a single target, they will transform into a 
Shard of Destiny, leaving a patch of frost on the ground for the remainder of the encounter.
[Heroic] Word of Recall - When cast the previous Call of the Val'kyr command will be repeated.
[Heroic] 
Link Essence - Two players will be linked together and any damage taken will be shared with the linked partner at 60% effectiveness.
Kyra Swing Timer: 1.5s (Curse of Weakness 1.8s)
Signe Swing Timer: 1.5s (Curse of Weakness 1.8s)
Skyja Swing Timer: 1.5s (Curse of Weakness 1.8s)
Your speed bonus from Provoke is very useful to move the boss(es) out of the damage reduction from Annhyldes Bright Aegis.
Sigil of Silence can be used to silence the Formless Mass adds and bring them into melee range.
Pierce Soul and Unending Strike stacks can be removed with Blessing of Protection or Divine Shield.
Blessing of Spellwarding can be placed on another player to allow them to solo soak Arthura's Crushing Gaze.
Soulful Blast can be reflected with Spell Reflection.
Stampeding Roar is useful here to allow players to move out with Daschla's Mighty Impact or to get to their soaks for Aradne's Falling Strike.
Grip in Formless Mass adds when they spawn so they can be cleaved down. If Mind Freeze is on cooldown, you can stop their cast with Asphyxiate.
Agatha's Eternal Blade
Brynja's Mournful Dirge