Glass-like shards that are the remains of windows and glassware from cities that have been destroyed by Chaos far in the grim dark future of Warhammer 40,000, Flects are powerfully addictive hallucinogens. Gazing into them causes visions, although repeated use seems to trigger mutagenic effects as well.
Flects are profoundly illegal in the Imperium. The Inquisition ceaselessly hunts both those who use and those who sell the drug, and not without good reason: the visions and mutations are gifts of Chaos, and Chaos does not give gifts for any reason other than to advance the cause of Chaos.