A Sorceress can actually cast spells from the opposite school of magic. Not every magical unit can do that. But what impresses me most in the expansions is the goblin knights. It turns out that there is no way to find a weakness to destroy them: neither with magic (counter-magic), nor with arrows (stone skin), nor with speed (ignores the first strike). All that remains is brute force, which lacks any strategy. But I always try to include them if I play as Chaos!