The Illegitimate Flame: Bride of Ashes - Chapter 74- Candy

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates

[WARNING: MATURE CONTENT]

Junpei Takeda, a high school student, is childhood friends with Noriko Tatsumiya and is bullied by Kido Shouta. One day, while bullying Junpei, they (Junpei, Tatsumiya, & Shouta) were suddenly summoned to a different world by the whim of “God”. Summoned Heroes with cheats, or so he thought. But it turns out Junpei’s stats were even worse than an average farmer. Tricked into a deathly labyrinth by the only one he trust, he walks the journey for revenge.

Between heaven and earth lies the greatest truth, and the manifestation of the great truth descends in the form of Gu.　　People cultivate the Qi of heaven and earth around their bodies to control Gu and exert their magical power.　　One year after his transmigration, Chen Qing finally discovered his special features on the day he contracted his first Gu. He was actually able to see through everything about the Gu...　　Therefore, he began to march toward the road to immortality step by step.