A conspirator is a person who takes part in a conspiracy and a conspiracy is a plan done in secret by a group of people to do something unlawful or something that will cause harm to someone.
An example would be:
-"Under the influence of Aurelio Saffi he became a Republican, and was arrested as a Mazzinian conspirator in 1874."
-"His departure was accelerated by a decree of expulsion as a conspirator."
sentence.yourdictionary.com