conspirator

Concise Oxford English Dictionary © 2008 Oxford University Press:
conspire/kənˈspʌɪə(r)/

▶verb make secret plans jointly to commit an unlawful or harmful act.

■ (of circumstances) seem to be acting together, especially with unfortunate results.

– derivatives
conspirator noun,
conspiratorial adjective,
conspiratorially adverb.

– origin ME: from OFr. conspirer, from L. conspirare ‘agree, plot’, from con- ‘together with’ + spirare ‘breathe’.