[Jack_Trowell]

From what is known, the Baron is a fighter-type leader, with no spells but a stronger potential early in the game.

Some units, especially the leaders, can cast special rituels transforming them to a new unit type. It is know for exemple that with enough hands of glory a necromancer could become a vampire or a lich.

The vampire can feed on villages to reduce the insanity gained by casting necromantic rituals (or is it onlye "raise dead" that give insanity ?), while the lich is supposed to be immune to the insanity effect/status (but I think that you need a temple or some other terrain to cast this ritual).
The necromancer apprentice could also use similar rituals to become a full necromancer (and later a lich or vampire), but the cost in ressources would probably be huge.

From what I understand some non casters leaders might have ability to those rituals, the Troll king is supposed to be able to call its Mum who is a caster while himself isn't.