Fomori are vile servants of the Wyrm, 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 their method of creation.
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.
They may have some connection to Fomorians.