Unnaturally large mouths is one of the creepiest visuals I've seen in horror. Buttloaf_builds gives us a good example of this in his model featuring a devil consuming some poor soul. There are a lot of cool techniques to notice here, including the Bionicle Rahkshi heads for hooves and the curved snakes. Of course, the devil's eyes are also very unsettling. Check out Buttloaf's Insta or Flickr for more!
