Dusk

"'Dying still really freaks me out, in the Dusk, looking over the Dunes …’""‘What dusk?’ asks Holly. ‘What dunes?’""‘The Dusk,’ Arkady says, ‘between life and death. We see it from the High Ridge. It’s a beautiful, fearsome sight. All the souls, the pale lights, crossing over, blown by the Seaward Wind to the Last Sea. Which, of course, isn’t really a sea at all...' -The Bone Clocks"

The Dusk is a metaphysical substance described in The Bone Clocks. According to Xi Lo (as Jacko Sykes), "if it touches you, you cease to exist." Dusk exists in a place visited by souls upon death.