Game Rules Guide

Damage is the rounded result of the casualty rate of the troops the Hero boosted, multiplied by maximum HP and by 0.5. On attack, those are the troops it marches with. On defense, they are all your troops defending where it stands, from any of your villages. If 10% of those troops die, the Hero loses about 5% of maximum HP. If none of your troops defend where the Hero stands, it loses no HP there.