Sesange Nappeun Yongsaneun Eopda

Hyeonseok Choi finally becomes the undisputed heavyweight MMA champion of the world, but gets hit by a truck, and dies. A fairy, Rahel, appears and convinces him to fight as a hero in the afterlife, but accidentally sends him to the side of the demons. Stuck in the demon world, he sweet talks his way into positions of power, but slowly discovers this battle between good and evil isn't as clearcut as it seems.
(Source: WEBTOON)