Darkness is created from intense negative emotions from a human. Humans are the only beings who can create and manipulate the Darkness, it can be quite poisonous to Spirits. When enough Darkness accumulates in a human their appearance is distorted by the Darkness turning them into Demons, however if Darkness is concentrated enough it can give twisted life to objects, plants, or even animals. Once someone completely becomes a Demon they can no longer use Mana. In times where a regular human cannot see Spirits, Demons always can.
Once a Spirit is poisoned by enough Darkness, they become a Fomorian. They no longer maintain any trace of themselves and become warped and twisted versions of their true form. They run purely on the instinct to kill and are completely incapable of reason. There is currently no known way of restoring a Spirit once they become a Fomorian.