The IBM or black ghosts. A Black humanoid made from a 100 percent transparent substance in an Ajin's body. Eriko understandly fainted from seeing guards get slaughtered by something she couldn't see.
Kei's is especially creepy. It keeps repeating whatever Kei ever said previously in life. All IBM can do this, but he's the prime example.
Satou alone might as well be the physical embodiment of Nightmare Fuel. He cares nothing for the plight of his kind, takes pleasure in the mass slaughter he causes, utilizes a large array of weapons to do so, has military training and combat experience courtesy of serving in the U.S. Armed Forces, is a superb strategist, has possibly the most experience with an IBM to the point where he can still control it even if hit with a tranquilizer, so long as he called it out before getting hit, and as of yet there is nothing that can kill him and keep him from regenerating. Even by beheading. To say nothing of his ability to improvise with the weapons at his disposal, like building a makeshift EMP, or drop a skyscrapper on his targets.
Season 1, Episode 3:
Kei slitting his own throat in front of Kai to heal himself.
Season 1, Episode 5: But when it comes to it, you still wanting to help you... You're the worst kind of trash...
Kei's torture throughout the episode, in each he is killed repeatedly while wrapped in cloth barely able to see out of it. One of his deaths is drill being ran into his teeth.
The casual attitude that the surgeons are showing throughout the whole experiment/torture session.
Tosaki threatening Izumi that she do her job or else would end up like Kei.
Season 1, Episode 6: I'm gonna kill you too
Satou threatening to cut off Kei's head and have him watch his own body grow one back.
Season 2, Episode 6: Must be tough being a lapdog
The drug that paralyzes one's diaphragm, making the victim constantly feel like they're suffocating.
