Nightmare Fuel / The Unexplored Summon://Blood-Sign

• The Black Maw that Swallows All: a living vortex of black slime, the inside of which is a mouth full of sharp fangs. If a summoner breaks certain rules of the summoning ceremony, their Material will transform into the Black Maw and eat them. Afterwards, it turns back into the vessel, whose mind will be completely destroyed. Even worse, it's revealed that it's actually the White Queen. Its real form is identical to the White Queen (except with the colors reversed), with all of her power and cruelty.
• To force Kyousuke into getting involved, the White Queen leaves over three thousand pieces of her body in Higan. They're too numerous and far too close to her blood vessels and vital organs to be removed surgically. Over time, they would have applied increasing pressure to her organs, killing her in a matter of hours. What makes this more terrifying is the fact that none of the pieces had initially damaged anything vital, showing the incredible precision of the Queen.
• The appearance of the Rainy Girl, the ghost who drives the plot of Volume 2. She's an elementary-age girl in a bloodstained raincoat, whose face is obscured by an umbrella. Her face is never actually shown (except when she's still alive), which is fortunate, given that she was killed by being stabbed in the head, nineteen times.
• The Girl's Backdoor, a mind-control device:
  • It allows the user to exert complete control over any girl between the ages of 10 and 25, and they can control up to three at once. The victims can't control their actions, but they remain fully aware of them. And there's more than one - the villains of Volume 3 deliberately distribute copies of the device to random people, to stir up chaos.
  • One user of the device is described in detail, Youji Makihara. He's mentioned to have had more than ten "girlfriends", despite otherwise being a normal student. Most disturbing is that he has often been involved in terrible accidents, such as his house burning down or a landslide killing his classmates, all caused by his "girlfriends". And he made plans to aim higher, by controlling particular people to potentially gain access to celebrities and other high-status people. Thankfully, Kyousuke and Isabelle find him, confiscate the Girl's Backdoor, and leave him with the girls that he was previously controlling.
  • And there's more! The victims of a Girl's Backdoor can still use it to control others, allowing the original controller to potentially control as many people as they can keep track of. On top of that, every copy of the device contains one of the White Queen's hairs - fundamentally, they use an Eldritch Abomination's power to terrify victims into submission.
• Volume 5 shows that the Secret War was a definite example of this. The White Queen's presence summoned a massive horde of Materials, which swarmed over the land and crawled up form the sea. Each and every one being completely invulnerable to non-magical means of attack, and their sheer numbers allowed them to overrun an army of summoners. But all of this paled in comparison to the Queen, who fired countless lasers that annihilated even the land itself.
• The White Queen is absurdly powerful, capable of instantly annihilating any opponent. She is also a unique existence, only capable of being summoned by one person at a time (unlike other Materials). This is completely turned on its head in Volume 7, where the White Queen gains the ability to turn human vessels into copies of herself with a bite. Imagine a Zombie Apocalypse, where every single zombie is an unstoppable Eldritch Abomination.
• After a summoner and vessel lose a summoning battle they are left in a near catatonic state for more than 24 hours. They are completely helpless and unable to protect themselves from any threat. They cannot resist any external stimuli and will follow any simple instructions others give them. Their memories of that period are also mostly gone. The concept of being so helpless for a full day or more and simply waking up with a gap in your memories is already bad enough, but in Volume 6 we see this in detail as the Meinokawa Twins and Kyousuke are turned into the playthings of Azalea and the White Queen, respectively.
• To prove that you don't need the supernatural to fuel nightmares, in volume 7 we are introduced to the Hairworm, a Government Repliglass prototype. As the name indicates, it's an artificial parasite that can control its human host, and it was developed to force captured criminals to perform sting operations against their organizations without a plea bargain.
• Volume 8 shows the result of an experienced summoner being dropped into the middle of a conventional military force. The soldiers inside the Artificial Sacred Ground are confronted by all sorts of monstrosities. Even worse, since Materials do not show up on cameras and other recordings so to people outside the circle it looks like the entire unit has succumbed to widespread panic and is eliminating itself through friendly fire.
• Kyousuke investigates a cult-piloted submarine. Except the Queen is dead, so all the images have been crushed and scratched out, scattered across the submarine's dark, creaking floor. Then the hull cracks, and water starts flooding in...
• Volume 9
  • The prologue opens with the White Queen monologuing about how much she loved Kyousuke and how special an existence he was to her. Then she asks:
  Oh? You want to know why I keep using the past tense?
  Figure it out for yourself, you sack of shit.
  • The novel goes into the past of Kyousuke before the Miniature Garden while he was 'trained' by his father as a summoner, something only briefly aluded to before in earlier volumes. It turns out Kyousuke is not even the first attempt of Doctor S to create a summoner that could maybe control the White Queen: the first generation was raised in an isolated sterile lab and was deemed a failure. Kyousuke doesn't know what happened to them, but he presumes they couldn't handle the outside world and its strees and were 'broken' by it.
  • Since the first generation broke because of not being able to handle stress, Doctor S' plans for the second generation, aka Kyousuke and his sister, involved raising them in a regular home while applying stress to prepare them as guinea pigs, which included sadistic training of Kyousuke and his sister in order to make them learn to excel at the summoning ceremony. Kyousuke had to follow insanely complicated rules based on the Summoning Ceremony on how objects in the house should be placed and angled in relation to each other. The worst part of this is that he wasn't told any of these rules. Doctor S would suddenly beat him with anything close that he could grab if he did something wrong, and Kyousuke would be forced to try to protect himself as best as he could and slowly figure out what he had done wrong to learn the rules.
    Kyousuke: He never told me what exactly he was hitting me for, so I had to memorize it all. He could turn anything into a weapon if he wanted to. When he grabbed something, I had to figure out how I could protect myself. I had to make sure I was hit hard enough to satisfy him but not enough to seriously injure me. But I couldn’t get carried away and fight back. If I did that, the beating would be two or three times worse than normal. The only way to survive was to make him feel like he had won.
  • Doctor S eventually figured out a more effective way to 'teach' Kyousuke once Kyousuke stopped crying even when he beat him: he started going after his mom and little sister. Kyousuke admits he really thought about killing his father after this started.