I’m leaning towards “Antipope”.
“An antipope (Latin: antipapa) is a person who, in opposition to the one who is generally seen as the legitimately elected Pope, makes a significantly accepted competing claim to be the Pope, the Bishop of Rome and leader of the Catholic Church. At times between the 3rd and mid-15th century, antipopes were supported by a fairly significant faction of religious cardinals and secular monarchs and kingdoms.”
One must distinguish between the sin of heresy and the crime of heresy.
A Pope does not lose his office by falling into the sin of heresy.