Thornmail's unique passive Thorns doesn't work on spells or items. Instead, it can only reflect auto-attack damage. When you hit an opponent, the passive will deal 10 plus 10% bonus armor as magical damage. It will also apply a grievous wounds stack that lasts for 3 seconds. If a target is also immobilized, the debuff will be slightly stronger (60% grievous wounds instead of 40% grievous wounds).