Hwansaengjwa

Abyss, an alternate reality created by a bored god, plucks humans from earth and sets them against monsters and each other in a sick gladiator game. Han-Su Kang, one of the last four survivors, is sent back in time twenty years as the last hope for humanity. He teams up with long lost school friends and enemies to relearn skills and arm himself with the knowledge to break the game once and for all.

(Source: WEBTOON, edited)