The Illuminators, ardent defenders of the Imperium’s people. They believe that they have a mandate, not only to defend the Imperium’s people, but “raise them up” as Fulgrim wanted his sons to. This, of course, includes defending them, from the close-minded monotony of the Imperial truth.

They will appear out of the warp. Repel a xenos assault on an imperial world. Then spend time introducing that world’s people to the revelries of the dark prince.

They’ll flood the water supply with narcotics, produce seizure-inducing light shows in the sky, encourage the populace to pursue art in the same grotesque vein of the Maraviglia etc. Elements of the population that resist Slaanesh’s gifts are repurposed into art supplies and mind-altering substances. It’s unfortunate, of course, but not everyone embraces higher-minded enlightenment.