I read this a long time ago (2019/2020) so I might be getting a few different manga/manhwa mixed together.
I believe it was a sudden apocalypse where there were these purple(?) spheres that randomly appeared across the world, from really small to massive.
I believe creatures came out of them - so the main group of characters had to train in the army to fight against them. I think they were high school students. They used modern weapons, like semi automatic guns etc. Definitely no magic.
The manga/manhwa was quite dark, with themes of bullying and death. I read it all and I know in the end everyone dies in the main group apart from one annoying guy.
I believe the manga was black and white, but I must have seen the cover at some point to believe the spheres are purple.