The vessel is an actual ghost ship the ghost of the god of a ship that sank and rotten at the bottom of the sea. The ship long since vanished but in the unlit depths of the ocean
When there is no sun they can pass between Creation and the Underworld which is permanently beyond the Sun's gaze. The Unconquered Sun doesn't just shine down on Creation he sees everything his light falls upon. The deep ocean is pitch black and often home to a myriad of horrors that at night rise up and walk upon Creation's shores. Such places as the darkest forests and the deeps of caves act as haunts where the things that cannot live under the Unconquered Sun's light lurk within Creation. In these places the boundary between Creation and the Underworld is blurred for there is no difference between day and night and ghosts can pass easily from one world to the other with the right Arcanos. Such is the instability that the deaths of the Primordials wracked upon reality. In the dark northern winter ghosts can walk freely for months at a time. The further north you go, not only does it get colder as the days get shorter, but ghosts can stay for longer in Creation.
Around the Pole of Fire the sky is blanketed by and endless cloud of smoke.
