Calculating Damage. Each weapon, spell, and harmful monster ability specifies the damage it deals.
1. Roll the appropriate damage die or dice. The damage will be a certain type.
2. Add any applicable modifier. For weapons, this is the modifier of the ability used for the attack. The damage type is the same as above.
Round down on the damage total, even if the fraction is one-half or greater.
Damage is subtracted from the target's hit points. This has no effect on its capabilities until it drops to 0 hit points.