Darkness is one of the two infallable elements, alongside Light.
Darkness refers to elemental phenomenon accociated with healing, rest, sleep, incapacitation, death, equalibrium and communication. It is accociated with Nocturna and demons, who used Darkness to help the souls of the deceased remain at peace and allow them to sleep (something that's impossible to do without Nyctomantic remedies unless one has a mortal vessel).
Although Darkness is accociated with death, it cannot actually kill or cause harm. Rather, it's connected to death and rebirth, and was once a viable connection to the Underworld. Before the Unrapture, Darkness was the elemental foundation for communicating with the dead beyond the veil.
When Darkness is used as Magick, it's called Nyctomancy.