For untold millennia, the Jailer patiently unfolded his plan to reach the heart of the Sepulcher. Now poised upon the precipice of his final victory, the heroes of Azeroth must rally to prevent the Jailer from dominating all of reality.
Overview
The champions of Azeroth assault the Jailer as he attempts to use the Machine of Origination to unmake existence.
The Jailer starts the battle by asserting control over the machine's Control Pylons using Relentless Domination.
Once all of the Control Pylons have been brought under the Jailer's control, the Jailer begins an Unholy Attunement in an attempt to activate the machine. The force of Shattering Blast can disrupt pylons and delay this attunement.
When the Jailer connects a line of Control Pylons to the center of the machine, The Unmaking begins. The Machine of Origination begins to drain Azeroth of its lifeforce, while the Jailer assaults Azeroth with Desolation. Once Azeroth's life force reaches 10% the Jailer will execute Azeroth and unmake all of existence.
At the start of the encounter, a combined in-game cutscene/pre-rendered cinematic plays.
The players run into the Heart of Eternity, where the Jailer stands waiting in front of a globe of Azeroth, looking at the map of Northrend.
Zovaal says: Can your mortal minds fathom...
Zovaal says: how long I have waited?
Zovaal says: Every event set in motion.
Zovaal says: Every pawn put into play.
The cutscene transitions to a pre-rendered cinematic, opening with a shot of Icecrown Citadel below the shattered sky.
Zovaal: All to claim the power slumbering within your world.
The camera slowly pans down below ground to the Forge of Souls.
Zovaal: The soul of Azeroth, wounded, vulnerable, yet teeming with potential.
The dormant soul grinders light up and start spinning rapidly. The camera changes to a shot of the Spire, lit up by a yellow glow from the bottom of the citadel. Suddenly, a massive beam of Azerite energy shoots up along the Spire, out of the top of the citadel, and into the top of the Sanctum of Domination looming in the rift above.
Zovaal: Potential that I will use to eradicate this flawed reality.
The cinematic transitions back to an in-game cutscene. The Jailer turns away from the beam of Azerite shooting into the chamber, turning to face the players and materializing his hammer in his hand.
Zovaal: Death claims the soul of your world. Now bear witness, mortals, to eternity's end.
In the Forge of Souls
Azerite shooting up along the Spire
The beam between Icecrown and Torghast
Encounter
Aggro
I will claim what is mine!
Rune of Damnation
Fall to ruin.
Your fate is sealed.
Torment
Break!
Rupture!
Martyrdom
You cannot withstand my power!
Relentless Domination
My will dominates all.
Oppression
Die at my feet!
Phase 2
The power of the First Ones is now mine to wield.
Decimator
Decimation!
Extermination.
Shattering Blast
You will break!
Rune of Compulsion
Reality crumbles.
Phase 3
The Jailer casts Unbreaking Grasp, stunning all players.
For uncounted ages I plotted to break this flawed cycle. To forge a new cosmos... dominated under my will.
All players receive Boon of Azeroth, ending the stun.
Upon the Jailer's defeat, another pre-rendered cinematic, "The Jailer's Fall", plays:
The Jailer falls to his knees and grunts in pain as the visor on his helmet starts to melt away like smoke. Images briefly flash by of the rift above Icecrown Citadel and the triumphant Jailer in the Crucible after the death of the Arbiter.
Zovaal: This must not be...
The screen flashes to white and changes to showing a foggy memory of a chained Zovaal—not yet covered in Domination runes—being put on trial in the Crucible. The camera shifts to showing his point of view, looking up at Kyrestia the Firstborne.
Kyrestia: You have betrayed your duty as Arbiter, and shall be bound forevermore.
In the Heart of Eternity, Zovaal twists on the ground as more of his armor melts away. In the memory, the Winter Queen pronounces her judgment.
Winter Queen: Your madness has threatened the cycle of life and death entire.
Zovaal's skin starts melting away from his chest, hand, and face as well, gradually reverting to a lifeless metallic vessel. In the memory, Denathrius stretches down an open hand to Zovaal, then closes it into a fist.
Denathrius: You could have sought another way, brother. Imagine what we could have accomplished together.
The camera shifts to showing Zovaal from above, shackled on the ground as the shadow of the Primus looms over him from off-screen.
The Primus: Why, Zovaal? What drove you to this end?
Zovaal strains against his chains and looks up at the Primus. As he speaks, his facial expression changes from one of angry defiance to one resembling concern.
Zovaal: You preserve that which is doomed. A cosmos divided will not survive what is to come.
The camera shifts back to the Heart of Eternity, where the modern-day Zovaal finishes his own sentence, speaking the last three words as his final ones while the light fades from the eyes of his now lifeless vessel. The sigil lodged in the hole at the center of his chest loses its light, fading into an inert orb. On Azeroth, the light beam shooting up from Icecrown disappears while the rift above rapidly closes until the sky is once again intact.
Zovaal's armor dissolving
Kyrestia
The Winter Queen
Zovaal's body dissolving
Denathrius
The past Zovaal on trial
The lifeless vessel of the Jailer
The shattered sky closes
Trivia
In early PTR builds, the Jailer's Adventure Guide entry read: For millenia, Zovaal manipulated forces throughout the universe to place him in this position of power. At this final step, the heroes of Azeroth rally to fight a cosmic being who has never known defeat.
The Jailer
Relentless Domination.
Unholy Attunement in an attempt to activate the machine. The force of 
Shattering Blast can disrupt pylons and delay this attunement.
Desolation. Once Azeroth's life force reaches 10% the Jailer will execute Azeroth and unmake all of existence.
Tanks
Dominate any player in line of sight with 
Chains of Anguish receive any Physical damage taken by the primary target.
Damage Dealers
Rune of Damnation will damage allies unless the player is inside a pylon conduit.
Rune of Compulsion will throw themselves off the platform if their shields are not removed.
Healers
Misery and 
Rune of Domination inflict heavy damage to all players.
[Fractal Cypher of the Zereth Overseer]
[Bindings of the Banished One]
[Chains of Domination]
[Domination's Deliverance]
[Epochal Oppressor's Greaves]
[Forbidden Truth]
[Gavel of the First Arbiter]
[Legguards of the Ultimate Threat]
[Runecarver's Twisted Ward]
[Scars of Fraternal Strife]
[Worldkiller Iris]
[Zovastrum, the Unmaking]
[Almanac's Echo]
[Hardened Bones], death knight
[Haunting Apparitions], priest
[Lingering Numbness], monk
[Ready for Anything], druid
[Resolute Barrier], warlock
[Reversal of Fortune], hunter
[Viscous Ink], demon hunter
[Sepulcher of the First Ones]
[Mythic: The Jailer]
[Damnation Aviation]
[60R] Heart of the Sepulcher
Patch 9.2.0 (2022-02-22): Added. 
