The Lady of the Voids.
A type of Slaaneshi daemon that is a step above the average Daemonette. She is able to enter a ship, completely bypassing the Geller Field, by bursting through astropaths and navigators. Once on a ship she will cause most of the crew to go insane by singing her song. Some mutilate themselves, some follower her as she goes about the ship killing and causing chaos.