Heretics are the magic heroes of Inferno.
"Heretics aim to harness demonic forces to their bidding. While they tend to train themselves with warrior skills, they must also know enough magic to control their infernal allies. Heretics can often be recognized by the number of protective wards and markings they wear to ward off demonic attacks." RoE manual
As they are heroes of an evil town, this allows them to get experience on Altar of Sacrifice and in Grotto for creatures only.
All Heretics start with the secondary skill Wisdom. They cannot learn Necromancy and Interference , which means they can only acquire them from Witch Huts (if allowed), Scholars, Seer's Huts, Pandora's Boxes or Events.