Completely inexplicable alien beings that use human senses to take control of people and force them to kill themselves or others, and can be foiled by damaging the sense in question? The Doctor will be along shortly to clean them up I'm sure.
They infect their prey through the windows to the soul. If you take into account the Cthulhu art and how everyone seems to see something different, they either have no shape or are projecting themselves into the real world. The closest we ever get to a clearly identified form of a creature, is a woman who claims to see her mother.
Also, everyones reaction is different, and can range from horror to awe struck beauty. The reaction the creatures want, seems to be whatever will get the victims next action to result in their death. Want someone to fall out a window? Appear as something super attractive outside a 2nd story of a building. Want someone to step in front of a truck? Show them something that traumatizes them to the point where they proactively look for ways to commit suicide.
Towards the end, the creatures pretty much just start imitating things and people in the hopes that something they do will get the person to take their blindfold off.
Also, interestingly, the creatures cant really exert that much direct influence in their environment. They dont seem to be able to pass the threshold of homes, and they cant open doors or windows. Instead, they rely on possessing the insane, and use them to carry out their will.
- Bonus: Both movies are in the Cloverfield multiverse.
They were simply horror fans who thought the monsters looked awesome and wanted to share their passion.
This WMG only applies to the book version. What if Gary was right that there really was not anything out there that was killing people, but people were so convinced that they were that they ended up really dying? The part that he did not get right was that uncovering the windows would cure them. It takes a lot more than that to remove a worldwide delusion such as what they are experiencing.
The creatures are in essence a very effective visual advertisement for suicide. Looking at them conveys information that would make a normal person see suicide as desirable. The "insane" devotees of the monsters represent people who engage with marketing campaigns and signal boost them for free, like sharing a "funny ad" in a group text or retweeting tweets from a corporate twitter account.
They invade the body through the Windows of the soul entering the body and taking control of it. In sane people they make them kill themselves, in insane people they make other people see other spirits or alternatively they cant possess crazy people but they can influenced them into forcing other to see them.
Problem is, angels are Eldritch Abominations too, therefore seeing an angel makes most people mad (or make them feel so bad about their own mundanity or guilty about their sins that they kill themselves), thats why crazy people sees them as beautiful, as crazy people dont feel remorse (thats why the term "criminally insane" exists).
Thats why sane people kill themselves and sane people start acting as functional individuals even planning, deceiving, driving and the like.
Creatures so terrifying that they make people go insane to suicidal lengths just by looking at them?
Whatever Saya actually would be is implied to be so hideous that just looking at them once will drive the victim crazy. Usually they are shortly killed after by Saya, but they are left crazy and suicidal nonetheless. The ones that have already gone crazy before encountering Saya? They see her as beautiful or fascinating.
This is assuming you chose Ending 2, in which Saya "blooms" and has her spores turn the rest of humanity into the rest of her kind. By that time, the "epidemic" would have 100% consumed Japan, but dandelion spores don't travel so far so fast, so of course it would take a bit longer for it to reach the rest of the world.
Since pretty soon every horror franchise will be part of this universe I'm gonna get ahead of this one.
It starts just after cabin ends and the creatures were either created by or came after the giant hand as part of a series of monsters that pray on one of the five human senses.
Possibility is that "it" is an experiment slipped from an experiment of the Department of Mysteries. Perhaps an hybrid of boggart, basilisk and fwooper.
- Or maybe a mix between boggarts and dementors, that's why they have characteristics of both; take the form of your biggest fear or your biggest pain, and make you crazy.
The thing was sent to pull humans to close the eyes... when the Weeping Angels arrive there will be the end of the world.
H. P. Lovecraft's influence is obvious. The creatures are Great Old Ones who are often described as having deranged followers and turn sane people crazy if ever seen.
Unlike in the book, the creatures in the movie are unambiguously malevolent. However, despite this, they never attempt to break into people's homes or forcibly overpower them in any way - even at the climax when they have a literal child completely at their mercy, they don't try to tear off her blindfold, but instead use their Voice Changeling abilities to try to trick her into taking it off herself.