Kakurenbo

In the game of O To Ko Yo, kids will disappear.

A 25 minute short about a game of Hide and Seek? That has to be fun for the kids, right? Wrong.

Children wearing fox masks gather in an abandoned city after dark to participate in a ritualistic game of Hide and Seek for their own reasons; one boy is looking for his younger sister, and a silent participant in this round sure looks a lot like her. The rules are simple -- if you lose, the demons will take you away. And as the night wears on and the streets light up, it becomes clear that the chilling rumours are not exaggerated...

Tropes used in Kakurenbo include:

All Myths Are True
All There in the Manual: The official website is the only place where you'll learn everyone's names--only four are revealed in-movie.
And Then John Was a Zombie
Break the Haughty: The most obnoxious player who acts all high and mighty? Reduced to a blubbering, terrified baby who actually faints in his final scene alive.
Cool Mask: The fox masks count, to some extent.
Creepy Child: A few of the players, including the Sorincha-imposter, and Hikkora at the end.
Creepy Twins
Cute Mute: It's hard to say if she can really be called "cute," since her face is covered by a fox-mask, but the lone female player doesn't speak, and her mask/costume are pretty cute...Neither the muteness nor the cuteness lasts.
- Also the twins. They are not very cute, however.
Demonic Possession: What the Fifth Demon is doing.
Evil Is Not Well Lit: In the city, the darkness is a constant backdrop to the events that take place. Of course, light isn't exactly any better...
Evil Sounds Deep: The reason why the girl doesn't speak until the end of the game.
Executive Meddling: In the original script, the losing children were killed and horribly devoured by the demons, but the higher-ups thought it was too horrific even by the short's standard...Ironically, the ending they did approve of was more disturbing.
Ghost City
Glowing Eyes of Doom: Subverted with the twins. Played Straight with the demons.
Hey, It's That Voice!: The dub contains voice actors from numerous companies, including one boy played by Veronica Taylor. No, he doesn't catch anything.
- Not to mention Dan Green as Yaimao.
Hunting the Most Dangerous Game
Ironic Nursery Tune: Well, it's not a nursery rhyme, but what else can we put this under?

"Reeeaady or noooot...'Here I come."

Last Stand: The twins make one against the demon pursuing them, choosing to go down fighting when they're cornered.
Living Battery: The machines use the children they captured for this.
Mecha: The demons' designs are partly based on this.
Ominous Fog: Appears at the end.
Parental Abandonment: Seriously, do any of the kids' parents know where their kids are going, what they're doing? Do they even have parents?
Powered by a Forsaken Child: What happens to the children who play the game and lose? Their bodies are taken to the center of the town and are plugged into strange machines--their Life Energy is used to light up the town.
Red Eyes, Take Warning: Subverted with the twins. Played Straight with the Demons.
Scenery Porn: To the point where the other wiki's article postulates that the theme of the work has something to do with the urbanization of Tokyo.
Senseless Sacrifice: Yaimao attempts to buy time for Hikkora near the end as they flee. Senseless in that Hikkora then chooses not to run.
There Are No Adults: No adults are seen or even referred to in this short--no, no one even mentions having parents. (Early concept art shows that originally, there were going to be adults in the town, but any adult who came in to find their children was turned into a featureless, mindless zombie.)
The Speechless: The twins.
Vicious Cycle: The losing children are used to light up the city. The light is used to attract new children to the town to play the game.
- The "winner" is possessed by the final demon and used to lure in more children.
Voice of the Legion: What It sounds like.
Youkai: The demons' designs are based on traditional youkai, for instance the Fifth is meant to be a kitsune.