Kantan na Monitor desu - Reaper Game 01: Decision

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

Kaidan (Kaidan refers to eerie tales of strange, mysterious, rare, or bewitching ghosts and monsters from Japanese folklore.) feed on cognition, wield information as weapons, and use information as currency, existing between the peculiarities of neither life nor death.They exist in human rumors and dwell in the shadows of the world.“Does this mean I will become a Kaidan?”In ancient times, they were worshipped as deities, empowered by faith.“Then I am a deity!”In more recent ancient times, they were regarded as demons and fed on fear.“I will become the master of Kaidan!”However, as the tides of time washed over, they gradually vanished, leaving behind only texts considered to be delusions.“To restore the glory of Kaidan is an undeniable duty of ours.”But you are not yet a Kaidan, hmm, nor a person.“Well, becoming a Kaidan counts as success.”

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.