The Darkness is the realm of eternal nothingness, and the prison of Agnuha and his followers. It is thought to be a plane of pure void and disconnection to the Overworld. It has few other names such as the Underworld, or more commonly: Nuha.
It is thought that when one dies, they enter a grass field on a cloudy day with no hills or trees. They will come to face The Gatekeeper, who will judge them based on the life before they died. If the Gatekeeper accepts them, he will open the door to The Lightness. If he denies them, he will open the door to the Darkness. There is no in-between.