This Rune grants the bearer a Damage Reduction of 3 once activated for 10 minutes. Any attack which inflicts 3 or less points of damage to the bearer during this duration does no damage. Any attack that does 3 damage or above will have 3 points of damage reduced from its full amount to the bearer.

Thus, if the bearer where to activate their Rune of Iron Skin and then be hit for an attack of 10 magic they would only take 7 points of damage.

All other adjustment such as attunements are accounted for based on the reduced damage. So that in the case above if the bearer of the Rune of Iron Skin had an attunement to Ice and took double damage from Fire they would take 26 points of damage from the Flame bolt spell (16 flame minus 3 DR equaling 13. Then times 2 thus equaling 26 points of damage)

If the bearer is hit with an attack of their DR or below they simply say “Immune” to let the attacker know they did not take any damage.