OSE Advanced - skeletal undead on front page

Does anyone have a source or description of the undead depicted on front pages of OSE advanced books? The referee's tome has a lich summoning some hooded spectral undead, and the Player's tome has some similar looking undead crawl out of holes in the ground to attack the adventurers, but they don't look like any monster described in the books as such. The closest I can guess is wraiths, maybe?