The series of BBC will also feature the star presence of actor Doctor Who Paul McGann.
Dormer will play a former soldier who now owns a tavern, and forcing the heroes to question the notion of justice in Paris a city deeply divided and war - torn Paris .
The co-executive producer Simon Allen explained to Digital Spy : " We needed someone who could convey immense sympathy, moral outrage and pain while being a real threat for Musketeers and everything they represent."
"We were so delighted when Richard accepted the role.It is an extraordinarily powerful role in an episode that changes the game. The cast and crew are happy to work with him and between shots captivated us with his amazing repertoire of imitations. The man is a genius! ".