The Collaborative International Dictionary of English v.0.48:

Heresiarch \Her"e*si*arch\ (?; 277), n. [L. haeresiarcha, Gr. ?;
   ? heresy + ? leader, ? to lead: cf. F. h['e]r['e]siarque.]
   A leader in heresy; the chief of a sect of heretics. --Bp.
   Stillingfleet.
   [1913 Webster]