(From Book of Vile Darkness, pg. 178) When a group of powerful fiends were trapped on the Material Plane, they attempted to create more of their kind through magical means. The result was a swarm of little, eyeless, reptilian creatures that displayed some insectoid traits as well. As these creatures matured, they took on varied forms. None of them were loyal to the fiends that created them, but they were wholly evil nonetheless. Their creators called them kythons. Because of their origin, they have been called earth-bound demons as well.
Kythons have spread throughout the world, breeding rapidly. Their eggs look like wet, mucus-covered stones. They seem interested only in eating and propagating their species; they are entirely without mercy, pity, or compassion for any other creature. As they mature, each answers only to those more powerful than themselves, so slaughterkings are the highest kython authority—at least so far. Given time, the kythons may grow into new forms that are even more specialized and powerful.
Kythons speak a strange smattering of both Infernal and Abyssal, but only to each other, never to others. Broodlings do not speak, but understand the commands of other kythons.