Formerly Slaanesh's favorite Daemonette, now on his permanent shit list.
Created when Slaanesh had a fixation on dancing, she is literally the most perfect dancer ever (except for that one time in 40k when an Eldar Harlequin beat her in a dance-off). Supposedly a hermaphrodite
as others daemonnetes but this is likely imperial propaganda as she won't return any of their calls.
After Tzeentch baited Slaanesh into a war against Khorne and Nurgle that resulted in his biggest ass-whooping to date, he sat back on his throne fuming. Not about the loss, but about being humiliated in front of the entirety of Chaos because that's just how Slaanesh operates. Masque, seeing her master was sad, started dancing to cheer him up, intending to get him so horny he'd forget about the whole thing (having forgotten the key part of Interpretive Dance is "Interpretive").
Slaanesh thought she was dancing to insult him, and snapped his fingers to place a curse on her. She's perma-banned from any pleasure that isn't dancing, including sight, sound, sex, drugs, pride, and so forth. She can only tell the stories that make Slaanesh look badass, and only through dancing. No longer the best Daemonette ever, she's just a moving flag for Slaanesh. Note that no other HNIC Daemonette was ever identified, meaning it is possibly still Masque despite her..."disability".
So why is she so damn special anymore? She doesn't just play the part of Slaanesh, she draws anyone around her into doing the same. If she dances a story of Dechala carving through Bretonnian Knights, Grey Knights may find themselves prancing forward as if riding invisible horses before their entrails spill out of their Power Armor. If she dances the story of Slaanesh in 40k consuming the Eldar, the souls of Asur find themselves pulled into the gullet of her master.
On The Tabletop
Being unable to join Units, Masque is not that great a choice. If carried into an enemy Unit she can deal some decent punishment and play some games, but she's not tournament tier.
Counts as a Herald, but without any of the Herald benefits. Her sole purpose is debuffing a Deathstar Unit, otherwise she's even worse than in Fantasy.
A bit more survivable than in the other Warhammers, she now has a rolloff against opponents trying to hurt her where she can shut down their attack if he gets a higher result. She can also reroll her Hit rolls if she has the same Movement as her opponents. Her main ability is making enemy models within 12 inches move at half speed, so getting her near your opponent's models will makes them move like they're in tar for at least one turn.
Masque doesn't exist in T9A rules as of yet. There are no named characters in the game at the moment, but the most important abilities have either been incorporated directly into the army or through magic items that allow you to emulate the character. Masque however has no equivalent, meaning her model is just a standard Herald with a backstory.