Darkness

Darkness is a conceptual element representing low light levels and shadows.

Nature
In a magical sense, Darkness is considered something physical, with a negative moral connotation. It's also metaphorically used to refer to evil forces, such as Demons.

Uses
Notably, Triskén's body is formed of a mass--one, as Khi discovered, was formed of nonliving matter--that is referred to as "darkness," appearing as a black smoke

In addition, dark magic can be physically viewed as a black substance, such as when it is collected around mirrors.

Weaknesses
Mirrors can be used to collect dark magic. There is also a "world" beyond the mirror that dark magic can be filtered into it.

Users
Darkins and demons use darkness in their magic, and it comprises an essential portion of their nature. Wyverns are also beings of darkness, although whether they are demons or some other form of being is not stated.