heresiarch
Concise Oxford English Dictionary © 2008 Oxford University Press:
heresiarch /hɛˈriːzɪɑːk/
▶noun the founder of a heresy or the leader of a heretical sect.
– origin C16: via eccles. L. from eccles. Gk hairesiarkhēs, from hairesis ‘heretical sect, heresy’ + arkhēs ‘ruler’.
'heresiarch' also found in these Oxford entries:

