Based on the Shin Megami Tensei If videogame.
The setting is Karukozaka High School, which is mysteriously sucked into a void and emerges in the realm of Makai, a world of demons. Most of its students and school employees are trapped in the other world along with it.

From Dark Horse:
Misaki Saiki, a young woman with a troubled past, is a professional dominatrix in one of Tokyo's most exclusive S&M clubs. However, her real money is something she likes even less than being a dominatrix. Ever since childhood, Misaki has had the ability to see and communicate with ghosts, and that talent is put to use by the Livelihood Protection Agency, who pairs Misaki with Souichiro Kadotake, a martial artist who happens to be deathly afraid of ghosts. Using her gifts, Misaki is able to help troubled departed spirits resolve what is troubling them and allow them to move on to the afterlife. If all THAT isn't odd enough, Misaki is an albino and a virgin.

A girl with a gas mask attempting to fight back against the strange world

From Aerandria Scans:

The beautiful singer Lucille and his orchestra invite you to the city of fear where puppets hunt down humans. The puppets are playing an oratorio of magic and illusions!

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This manga also features the unrelated short story Camelot Garden in vol. 5.

A collection of stories inspired by Grimms’ Fairy Tales.

Story 1, G's Diary: A girl with no memory of who she is spends her days working in a house she has no memory of entering.
Story 2, Frau Trude
Story 3, The Summer and Winter Garden
Story 4, Little Red Riding Hood
Story 5, The Frog King or Iron Heinrich
Story 6, Little Brier-Rose
Story 7, The Bremen Town Musicians
Story 8, Rapunzel.

From MangaHelpers:
High-schooler Matsuzaka Satou has a reputation for being easy, but one day her lifestyle of sleeping with one boy after another comes to an end. It happens when she meets the child Shio, for whom she is convinced she feels true love for the first time. Satou may seem sweet and innocent, but there is nothing she won't do to protect their life together, including committing murder. But from where did she acquire the little girl, and how long can their "Happy Sugar Life" together last?

Included one-shot:
Volume 2: White Sugar Garden, Black Salt Cage (pilot)

Short story:
A short story was released in Young Gangan 23/24 of 2016 which is referred to below as 19.5. It was later released in Volume 6 as an extra.

A collection of 4 short stories with horror/disturbing/suspense theme

1) Shoujo Pandora
2) Beauty
3) Fish-man/Merman
4) Puppet

Count D has opened his petshop again, this time in Tokyo's Shinjuku!

For those who haven't read Petshop of Horrors, Count D's Petshop is a place where one can find animals from the most ordinary housepets to savage predators to mystical beasts that should belong only in legends. Customers often find that their new pets change their lives more than they expect, fulfilling their dreams, or realizing their nightmares.

Seven college students are coming back from their graduations. During the travel they are shocked to see a strange, cow-like creature with a human face. This accident means they only have seven days left to live. What will they do to stay alive until the fated day?

A spin-off comic based on the movie Sadako has appeared! Sadako, who doesn’t get to appear much in the film, was actually doing this and that?! Witness a short comedy from Sadako’s perspective! You may see a side of Sadako you’ve never seen before… ?

Five days after the sudden appearance of zombies... The protagonist, Masaru, who is having it rough surviving at school, is splendidly saved from this predicament by high schoolers Sekigahara Miki and Nakiri Karin. Though Masaru is at first relieved, he ends up realizing their true intentions...!!

A slit-mouth woman encounters a young boy she's unable to scare.

The protagonist is a girl with the ability to visit a surreal alternate dimension full of monsters in her dreams. One day, she encounters another human girl there and receives a plea for help.

While humans toil at their ordinary, boring lives, a battle secretly begins among a new species that evolved from humans. They disguise themselves as human children, and instead of maturing into adults, they are each gifted with a different "special power". They are the new humans, known as "Kodomo." Hinagata Heiji is a Kodomo who can bring any inanimate object to life by dripping his blood on it. In order to find his own feeding ground, he keeps transferring to new elementary schools.

Even if he forgets, I will remember

It centers on a girl named Wakaba Himekawa, who is aware that she is growing up slower than other people, but otherwise lives a peaceful life. On the same day that she finally gets her first period, her childhood friend Takaya confesses that he loves her. While Wakaba had never felt that way for her friend before, she is still happy, if confused. As she is about to answer Takaya's confession, an alarm suddenly sounds, and a broadcast begins that will change her everyday life.

Denzel Westwood is a young, talented magician without a care in the world, until one day his eyes suddenly begin to turn amber, prompting a series of events which will lead him to discover the truth of the world in which he lives and the mysterious Abyss which surrounds it. This is the story of a boy filled with light.

English version
Original webtoon

Part of the Umineko When They Cry franchise. This side-story takes place outside the main story.

The story expands on the use of the Purple Declaration introduced by Bernkastel in Twilight of the Golden Witch. The Red Truth still applies in this game even though there are lesser than usual games. Knox's rules are stated in red to show that it is valid in this game.

A collection of one-shots by manga-ka Tamura Yumi.

1) X-Day:
4 short depictions of tragic misunderstandings and their consequences.

2) 4nin no Onna:
A very famous, rich, old hotel owner is found dead and when the police come to investigate, his long-time maid says that her master was killed by 4 women...

3) Kanojo wa Dare wo Koroshitaka:
A woman is found covered in blood walking through the streets of Tokyo. When questioned by a policeman, she utters that she has killed someone. To figure out what happened, the officer begins to read her intriguing diary...

4) Hirotta Otoko:
The five members of Maki's family are so picture perfect that they even star in heart-warming commercials advertising food and a happy, loving family. But what are they really like beneath the surface? They're forced to find out when they pick up a mysterious guy named George, who they almost run over by car on their way to their winter lodge...

September 11, 197x, In the whole Kantou region a magnitude 8-9 violent earthquake occurred. Scholars called it the great destruction of the Showa Era, but the people called it the earthquake from hell! Volcanoes erupted, buildings crumbled, Tokyo became decimated in a single day. The government, after failing to recover the city, abandons Tokyo and moves the capital. The survivors form a society with no laws. In this harsh lawless world emerges Jack, a 10 foot giant who annihilates anyone and anything in his path. The manga details his journey.