Also the cults associated with the Chaos Gods in the Warhammer and Warhammer 40k universes. Most numerous are those dedicated to Tzeentch (or Tzeentch under a under different name). Slaanesh is also popular, especially with decadent aristocrats. Of all Chaos gods, Khorne is the least popular with cultists, as his worship demands frequent and messy sacrifice, and having people disappear overnight is somewhat uncommon in the Old World (though par for the course in the 40k universe).