"Harvest their immaculate muscle mass and provide our Prince with its precious product! My escape from the eternally enduring dance of the damned will be determined! I MUST HAVE THOSE MUSCLES, MAN!"The Masque is one of the Daemonettes of Slaanesh.
Official Canon Edit
The Masque was at one time Slaanesh's favorite Daemonette, she created to be the ideal dancer. But after Slaanesh lost a war against Khorne and Nurgle, the Masque attempted to console Slaanesh by dancing. Slaanesh thought that the masque was mocking them, so they cursed the Masque to dance for the rest of eternity without rest, and is unable to experience any other sort of pleasure.
That said, she is able to force others to become unwilling participants in her dance, until they eventually dance themselves to death. Only a single Harlequin Solitaire was able to break this spell by matching the Masque's dance without fail.
TTS Canon Edit
The Masque attempted to lift its curse by collecting the muscles of the swolest Catachan Jungle Fighters as a tribute to Slaanesh, but her plan was foiled when the Raven Guard and Salamanders arrived. Because gene-seed enhanced muscles wouldn't work for the ritual, the Masque complained that her plans were ruined. However, she was horrified to discover that Corvus Corax was among the Imperial forces, and returned to the warp in fear.