Weapon runes can be applied to weapons that possess a rune slot to provide permanent bonuses to that weapon's effects. They can be added to weapons at the Merchant's Guild in Hightown. Unlike Origins, once used they can't be removed, only replaced with the previous rune destroyed.
Runes which add damage (e.g. Rune of Fire) add 20% of the weapons base damage rounded to the nearest integer. This damage is not modified by damage modifying talents and/or item properties.