The Reaper(2022)

Pyo-Wol was an orphan who wandered around the continent until he bumped into a mysterious man and lost consciousness. When he wakes up, he finds himself in a dark, confined space with only one iron door for an exit. Wol survives the next several grueling months, trying to keep a cool head and stay in shape. When the iron door finally opens, he discovers he's not the only one in this miserable situation. A group of masked people has kidnapped 300 children to train them into human weapons. Wol must fight and kill the others, for only a handful of the strongest will be selected. Will Wol be able to become one of the chosen and find a way to free himself from his captors?

(Source: Tappytoon, edited)