It isn't often anymore that I come across a word I've never seen before, so when I found heresiarch while editing a project about early Christian theology, I got all giddy and excited.
Yeah, I'm a nerd.
Heresiarch combines word parts that you've seen before. The heres- comes from heresy, and the -iarch is the same as the ending of words like monarch, patriarch, and oligarch. It means "leader."
A heresiarch, then is the leader of a heretical cult.
We don't get to use this word so much anymore. In popular discourse, it has largely been supplanted by words like crackpot, wingnut, and politician.