The Masque
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). Thanks to being referred to exclusively as "she" in fluff (despite her half masculine chest) Masque is also the only canon fully female Daemonette.
The Legend
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 had a sad, started dancing to cheer him up intended 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.
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
Gallery
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The Masque's model.
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The Masque in the trading card game.
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The Masque in the 8e Daemons of Chaos army book.