Light or Dark magic.

One stone on the hero level is almost fatal, unless you've previously acquired a mill or there are red stones visible on the map.

Firstly, even on the hero difficulty level, one stone is not fatal.

Secondly, there is a clear imbalance between the factions on the hero level, which is why games between "squirrel-like" opponents are played on veteran difficulty. I read somewhere that the game was balanced around the Warrior class.