Spoilers: The curse and its cause. No campaign ending.

The Accursed Troupe
The Masque leads the Accursed Troupe in Immortal Empires. She is the free Legendary Lord added in Update 7.0, in the same season as Dechala’s paid Tides of Torment pack. Access follows Slaanesh ownership on the same store: the base game, or Dechala’s pack, is a route. The dancer is not a separate full-price lord.
Her campaign is the Eternal Dance. Continued campaigning fills a repertoire and changes what the army is good at. The fiction says she cannot stop dancing. The mechanic says the army should not stop either. A pause is a wasted bar.
She is a mobile daemonic melee lord, a Daemonette rather than a Keeper of Secrets. N'Kari wins by seduction and by catching isolated victims. Dechala wins by spending Thralls and draining settlements. The Masque wins by keeping the performance going until the bonuses match the next enemy.
The dance that was taken for mockery
Update 7.0’s account is the curse in plain terms. The Masque was Slaanesh’s most favoured Daemonette, a dancer of unmatched skill. Her performances told his glories. Then Slaanesh suffered a terrible loss to Khorne and Nurgle. She danced for him, to ease his mind, with the same skill she had always used.
He took the dance for mockery. The rage that followed condemned her to dance for eternity, without rest or respite. She did not abandon him. She and the Accursed Troupe still carry Slaanesh’s ruin into the mortal world, and the Eternal Dance is how that sentence looks to the people who have to watch it.
Slaanesh had suffered a terrible loss to Khorne and Nurgle, and he heard an insult in a dance that was meant as comfort.
Celebrations, cults, and a Khorne neighbour
The Tides of Torment answers describe where a cursed dancer belongs on the campaign map. She is drawn to celebrations and to places of excess. She arrives, the dance does its damage, and she moves on. That pattern is more destructive than N'Kari’s longer game of influence. Two cults are hers from the start, and they can reach toward the Dwarfs or the Empire. Yuan Bo’s Cathayan faction is among the nearby powers.
The same answers give her a hatred of Khorne that the curse explains. Skulltaker is named as a neighbour who can receive it. That is a campaign pairing, a direction for the opening war. It is not a story in which the cloak and the dance have already met, and it is not an ending.
Update 9.0 lets Archaon subjugate the Masque along with N'Kari and Dechala. Regions go to a Slaanesh vassal. The curse does not come off because the Everchosen won a battle in a sandbox.
The Masque of Slaanesh: gameplay and availability · More character stories
Sources
- Update 7.0: The Masque — Total War — totalwar.com
- Tides of Torment Q&A — Total War — totalwar.com
- Tides of Torment release — Total War — totalwar.com
- Immortal Empires DLC Ownership Guide — SEGA — support.sega.com
- Update 9.0 release notes — Total War — totalwar.com
- Update 7.0: Introducing the Free Content and The Masque — CA — community.creative-assembly.com
- Total War: WARHAMMER — A New Player Survival Guide & FAQ — CA — community.creative-assembly.com
- Update 9.0 Release Notes — CA — community.creative-assembly.com