The formori are created when a Bane possesses a human via a spiritual "hole" left by some form of sin or spiritual corruption. The Bane will slowly gain more and more control, until they are completely fused with the host and cannot be separated. Fomori powers vary wildly, but are usually based strongly on the type of possessing Bane and/or their method of creation. For example, a typical Bane of Lust will result in a type of fomor known as an Enticer.
Fomori may have some connection to Fomorians. However, the most common (although hardly the only) method of Bane infection is via Pentex; either through employment or usage of its omnispresent products, an unwitting individual can open themselves to possession. Their Gaian counterparts are called Kami, while the Weaver ones are called Drones and those of the Wyld are Gorgons.