Down-worlders and demons are two of the prominent groups within the Shadow world. Demons are creatures of pure darkness, who feed off the vitality of life until there is nothing left of a world but destruction and decay. It is never specified where demons come from, but since Shadowhunters prescribe to no particular religion, it may be that demons come from the various planes of evil, such as Hell and Naraka (the Hindu equivalent of Hell). The demons in City of Bones range from the weak and unintelligent ravener that attacks Clary in her home to Abbadon, the greater demon that is powerful enough to possess Madam Dorothea from another plane of existence. This suggests there is a hierarchy of demons and that the lesser creatures are like foot soldiers to the greater captains or generals.

There are many types of Down-worlders, and the most common are vampires, werewolves, warlocks, and faeries (nymphs, dryads, etc.). Warlocks and faeries are of mixed parentage. Warlocks result from the union of a demon and a human, where faerie origins are unclear but believed to be either the offspring of demons and angels or angels who were cast from heaven. By contrast, vampires and werewolves are created from demon viruses that have particular effects on humans.