During the Age of Myth these seers become obsessed with symmetry and many of its members went though the Doppelfleisch, a surgical biomancy that turns half of their bodies into a perfect replica of the other but cast in living metal. This practice is tainted by Slaanesh and the flesh side begins to rot, and they are forced to use pomanders and perfumes to cover up their corruption and the stench. The ones whose bodies reject the procedure try desperately to find a way to treat their broken bodies.
The Herald of Slaanesh Litheclaw promises them salvation from a hideous demise through the Dark Prince. They quickly accept the offer and carve enormous sigils of Slaanesh in the salt flats of Demia in exchange for their extended lifespans.
Tyrion hears of this betrayal and mobilises his armies to destroy the sigils. From the sigils emerge emerge thousands of Daemonettes that are eventually driven off, but not before slaying many of the Aelven cavalry.