[Reminder this is chapter "The Black Swordsman, Once More"]
After this tree, I started wondering about the true nature of these monsters.
Skull Knight said that spirits of the Interstice cannot be seen by unbranded people, so this is not one of them. Nor an Apostle, obviously. But... in the first volume Guts was attacked by possessed skeletons that killed the priest and his daughter, so, in a certain way, that world can interact with the physical one. Intriguing enough, I'd say.