Location from acotar-mist-and-fury by
An ancient subterranean fortress carved into a black mountain on the edge of the Night Court, the Prison holds beings so dangerous that even the High Lords of Prythian feared to kill them.
The Prison is not a place of bars and keys but of magic and will. Each cell is a pocket of stillness, sealed by spells older than the courts. The High Lord of the Night Court holds the keys—literally, a set of ancient keys that unlock the wards—and only he may enter or release what lies within. It is a place of last resort, where immortals are sent when death is not enough.
A black mountain with no entrance visible from the outside, its interior a labyrinth of darkness and silence. The walls are smooth, unbroken stone, and the air is cold with the weight of millennia. No light penetrates the deepest cells, and the only sound is the echo of one's own heartbeat.
Also known as: The Prison, the mountain prison, the black mountain, the cells of the forgotten