The Prisoner (2020)

Se Do, once a competent detective, is falsely accused of murdering his wife Tae Hee and is sentenced to life imprisonment. In the prison Se Do is staying, the warden puts up a secret MMA fighting tournaments between the infamous criminals and makes money out of illegal betting. Even killing the opponent is allowed during the match. Se Do finds out one of the tournament participant is the real killer of his wife and decides to join the tournament. Now his game to find justice on his own begins.