The phrase "chief villain" refers to the main or primary bad person or character in a story or situation. They are usually the one causing the most trouble or being the most evil. Full definition
Sinise's chief villain poses no threat whatsoever. (metro.co.uk)
The flurry of news about multiple films featuring Batman's chief villain shows just how desperate the industry is becoming. (theatlantic.com)
Halo was marketed extremely well, and painted a picture of a conflict between the two main protagonists that was sure to make the player see things from angles that, perhaps, they didn't expect and possibly even make master chief a villain of sorts. (gamingbolt.com)