It has been implied that the Ascians use a distinct form of teleportation magick that sends them through the rift between as opposed to the aetherial sea, so that could further imply that some of the terminus beasts are also coming from somewhere in the rift...which would still further line up with the minstrel's song claiming we would shine our light there at journey's end.
Considering that we've got places as unusual as Pandemonium buried within the world's depths, I would not be surprised if there's some realm between corporeal and aetherial that the beasts are coming from and I'm hoping where the post-apocalyptic looking dungeon takes place.