To unlock the special class, you must first purchase it in the Manor. This is the "path" you need to take in order to make it available for purchase:
Smithy -> Health Up -> Upgrade Knight -> Equip Up -> Enchantress -> Upgrade Mage -> Unlock Miner -> Upgrade Knave -> Potion Up -> Unlock Spell Thief -> Upgrade Spell Thief -> Invuln. Time Up -> Beastiality
Then simply select a character from this class once it comes up while picking your heir.