In the fictional world of Glorantha, a person who is Illuminated is metaphysically altered. Other people cannot detect whether they are chaotic. The deities the character worships seem to develop a "blind spot" with respect to the Illuminate's sincerity and faithfulness to the cult. The person may privately ignore various cult restrictions, may even attain membership in mutually hostile cults.
One becomes illuminated by exposure to Nysalori riddles, administered by a Riddler. This is most common in the Lunar Empire. Other ways include participation in Chaotic quests, and direct mental contact (i.e. magical) with other Illuminates.
Chaotic beings who become Illuminated often feel their destructive urges fade. On the flip side, non-chaotics who experience Illumination often come to feel some degree of acceptance of Chaos as an ambivalent tool, or a vital and necessary part of the universe.