Supernatural: Fallen Stars
The Dyrr Dottir is the prison of Night Mara or the Nightmare. It is the being that breaths life into your nightmares and makes them living torments. It is said that she is capable of giving one dreams so dark and real that they will stay with the dreamer long after they wake.
She is the cousin of death and has relation to the reapers in some way. There are very few things that can hurt her. Angel blades can harm and drive her off. Magic has been used to trap her, but only the Scythe of Death or a weapon of God constructed by a ritual on one of the Words of God can actually kill her, but that would come with severe consequences.