archconspirator

plural archconspirators

1. A chief conspirator.

   And the third archconspirator--and this is the bad one--the real archconspirator, this is the one with the pencil-thin, waxed mustache […]

   Records and Briefs New York State Appellate Division (page 2890)

   1 more example

   The jurors listened carefully while Eidem portrayed Fern Sankey as an archconspirator who drove her husband into crime and was intimately involved in all major details of Charlie Boettcher's abduction.

   2016, Timothy W. Bjorkman, Verne Sankey: America's First Public Enemy