Sekai de Tadahitori no Mamono Tsukai ~Tenshoku Shitara Maou ni Machigawa Remashita~ - Vol. 2 Ch. 9 - S Rank Adventurers

[Manga-Titan]

He's not the demon lord just an overpowered monster tamer since his stats from being a hero transfered over but there might be certain conditions to force ably change to a demon lord.

 

[CreateJunkMail]

Fist Fuck's head needs to be ripped out from his body, then smashed. Afterwars, the MC needs to do the same to the other two scumbags.