Some heroes can cast spells in battle, they help the mage they work for. The units that get hit by these spells have a chance to negate them, following the color resistances listed in the encyclopedia. Heroes that cast spells are:
The hero battle spells (Arcfire, Firestorm, Icicle, Chill Touch, Wall of Thorn, Turn Undead) do their damage at the begin of the battle, without suffering from reducing factors. Their accuracy=100%, their efficiency=100%, there is no random modifier, there is no fatigue. This makes the damage by hero spells very effective.