Same autor as above, same darkness.

Rape, blackmail, everything is pretty much mess up.

When I opened my eyes, the first thing I realized is that I am not a living person. Who am I? Why am I wandering around this place? And who is that man who visits my grave every week?

The story starts with a boy saying he had kill lots of people... Just the start is already dark.

A mysterious monster has appeared at school and has declared he will eat "one person a day"! Deciding the value of lives is the key for survival! What will be the fate of the protagonist once he becomes the designated food!?

Everything is kind of dark, and the art is really good!

Mafia and sexual abuse

It is simply good. + Murder

Very well known and dark manga.

A relationship between a crazy mother and his son

Sexual abuse

Horror, creepy...

Magical + gore

Is dark, not only in characters and story, but also in the art. Is... WOW, is dark.

Very well known and dark manga, such as Battle Royale. So famous it even has movies!

Is not super dark, but the tortures are something different.

A time loop that makes the same day happen again and again. The only way to go to the "tomorrow" is by... Killing someone everyday.

Animal Abuse, Bullying, Mature Themes, Nudity, Physical Abuse, Violence

People are trapped in a world of skyscrapers with not way down, and with the buildings connected by bridges. However, they are not alone, some masked people are their enemies.

The story itself is dark and strange. + Bugs

The art is simply beautiful, really really dark.

The extra story is really dark + sexual abuse

An island where people who tried to commit suicide are abandoned.

A guy decides to train in the 'arts' of torture to take revenge in the guys who bully him, who also happens to have done something VERY, VERY bad to him.

Survival island + supernatural 

Same as warehouse, but less torture and more... Mom complex?

Makoto made a deal with J in order to make his hidden desires granted. Then, he was suddenly reborn as a demon?!

Magical girls killing people is pretty mess up

Isekai but with a lot of explicit torture and revenge

Is short and not really popular, but it has a good story.

At the National Local Mascot Festival, children all across Japan can meet their favorite local mascots. But as the festival gets underway, it becomes clear that what's inside these costumes aren't people - they're something much darker, with a taste for human flesh. (But not really my cup of tea)

Everything is dark.

From an early age, Xu Shenze would often get caught up in the same strange dream. Over time, the dark shadow that the dream cast over his life grew, until, one day, the shadow became real, and suddenly Xu Shenze's life as he knew it was completely torn to shreds.

It's still on going, pure horror.

Violence and a bit of gore, it's from Webtoon. I think it also has a Netflix adaptation.

After finding out his sister has been gang-raped by a secret fraternity before she commited suicide, Lee Kirin decides to catch the rapists and take revenge on them.

Gangrape, I have to dropped because there was too much rape and humiliation, but some may like it! (Sex slave, toxic relationship)

One shot about human's cruelty to animals

Trauma, abuse, the feeling of being use... Not a lot of chapters yet, but it is dark and reading it will make you feel uneasy.

I find it kind of dark, even though I didn't finish it yet.

Good story, survival, scape from a place, short... Interesting backstories for characters.

Nine boys were murdered in the long past, however, their story seems to travel across time without clear end. It's dark and with an interesting plot, not very long either.

12 girls are imprisoned in a mysterious prison.

Everything is mess up! It's similar to 'An Easy Target'

A rainy night. A mysterious island. A boy with a dangerous story to tell – if only he could remember who or where he was. So begins a chilling journey into the unknown – when uncovering a twisted world of secrets is the only way to survive.

One of the characters loses both of his arms, there is sexual abuse and all kind of dark things. The sequel is kind of worst to be honest.

Kipnapping, rape, violence... 

Short, bullying, murderer... Hope?