The Prison (2017)

- The plot is similar only difference is this happens inside the prison
- ML is an Undercover Cop but in the prison he comes to take revenge on those who killed his younger brother
- ML tries his level best to catch criminals before his identity getting exposed

I think the plot is quite similar to this movie
- ML goes to prison as Undercover for investigation
- Faces many obstacles & needs to survive in the prison with other criminals
- Finding truth about the investigation he's doing

Both movies have a cop undercover in a prison as the main character.
The Prison takes place almost fully in a prison while The Merciless skips around a lot.
The Merciless is definitely darker but it has a really good plot that makes you never know what is going to happen next.
Both are great movies.