Can you recommend some good horror manga?

--- Ibitsu by Haruto Ryo ---

SUMMARY: A boy went to take his trash out late one night, and found a strange, creepy, lolita woman sitting amongst the garbage bags. She asked if he had a little sister, and he answered her, hurrying afterwards back to his apartment. When he looked out the window, she was gone. Who is the strange woman, and why does she give him such a bad feeling?

Even though it's only 12 chapters, it's intense and very creepy!

--- Deguchi Zero by Seta Haruhi ---

SUMMARY: Akabane Yuuhi, a 15-year-old girl who aspires to become an actress. Well, her dream might soon come true. She has been accepted into 'Diamond Actors Academy', a very prestigious school which trains young boys and girls to become actors. Many famous actors and actresses graduated from this school. So, of course, Yuuhi is excited. But, things start to get a bit weird. Just what is up with their teacher, Otome Kasumi? The acting is getting a tad too realistic to be considered acting. Things are getting really creepy. Is the school really what it seems? Or is there more to this school than meets the eye?

It's thrilling and gives you a sense of dread when you read it.

--- Dead Tube by Yamaguchi Mikoto ---

SUMMARY: Distributing thrilling videos in secret!! A school game that puts life and death on the line!!

It's more of gore than horror. But still fun to read.

--- Franken Fran by Kigitsu Katsuhisa ---

SUMMARY: Fran can make anyone into anything, raise the dead, switch heads and bodies, and give you those eyes that you've always wanted. But do you actually want them? Is it a good thing to raise the dead? Do the ends justify the means? And does Fran care?

Also a lot of gore. But also a lot of biology! This one is really fun to read!

--- Kamisama no Iutoori by Kaneshiro Muneyuki ---

SUMMARY: Takahata Shun's day at high school begins just as normal and boring as ever, but it doesn't end that way. After his teacher's head explodes, he and his classmates find themselves forced to play children's games, such as Daruma ga Koronda (a game like Red Light/Green Light), with deadly stakes. With no idea who is behind this mysterious deadly game session, and no way of knowing when it will finally end, the only thing Shun and other students can do is keep trying to win...

Lots of gore and reminded me of the movies SAW. You need to read both this one and the 2nd book Kamisama no Iutoori Ni to fully understand the story.

Edited to add:

--- Online: the Comic by Keropyon ---

SUMMARY: Mai Yashiro (22) was an average office worker until the day a video game console arrived in her mailbox. The game she is forced to play, “Nightmare,” is a dangerous game in which one risks both the lives of the ones they love and their own body!! If you refuse to play, those dear to you die. If you lose battles in the game, parts of your body (your arms or legs, your voice, etc.) cease functioning. The horror of this game spreads throughout society in a flash, and, to fight it, Mai’s company creates a “Nightmare” Strategy Team. After being assigned to this team, Mai and her dependable comrades boldly rise up against this hellish game.

This is seriously good and underrated. The art and plot is really great with lots of mystery and horror factors. It's also very unique with its gaming features. Definitely recommended!

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I'm not including popular ones like Yakusoku no Neverland, Kiseijuu, Higurashi ni Naku Koro Ni, etc.

The Dreaming by Queenie Chan

Bizenghast by M. Alice LeGrow