heresiarch
The founder of a heresy, or a major ecclesiastical proponent of such a heresy.
He died in 1223, and seems to have had no successor; not is it an ascertained point that more than one such hæresiarch was ever recognized.
Where it comes from
Borrowed from French hérésiarque, from Ecclesiastical Latin haeresiarcha (or directly from the Latin word), from ecclesiastical Byzantine Greek αἱρεσιάρχης (hairesiárkhēs, “leader of a sect”), from Ancient Greek αἵρεσῐς (haíresĭs, “heresy”) + -ᾰ́ρχης (-ắrkhēs, “leader, ruler”), corresponding to heresy + -arch.
Adapted from the Wiktionary entry heresiarch (CC BY-SA 4.0).