Don't really know what you're after but as for magic:
- Necromancy, creating undead. Vampires. Turning your sovereign into a lich. (some of this about to be modded already)
- Rituals demanding several turns of uninterrupted casting, with truly global results, e.g. wrath of the (sovereign), meaning a chance for a rain of fire over all enemies at war, on the cloth map, each round ... Breaking the Law of Life: for the next 50 turns all killed units become undead ...
- werebeasts, spreading lycanthropy
- polymorph spells, on self and on others
- building on Wintersong's blood magic mentioned above: also sacrificial magic (like in Dominions): round up some local virgins and unleash your wrath ...
I realise it must seem like I'm crazy ...