A damage bonus item property on a weapon or ammunition increases the amount of damage inflicted in combat. Each bonus has an amount and a damage type.
A weapon can have multiple damage bonuses; those of different damage types stack, while those of the same type do not. For the purposes of stacking, an enhancement bonus is considered a damage bonus of the weapon's base damage type. (In the case of multiple base damage types, an enhancement bonus is considered to be only a single damage type, in this order: slashing, piercing, bludgeoning.) After stacking is determined, all physical damage bonuses are converted to the weapon's base damage type.
A damage bonus is distinct from an on-hit property that inflicts damage. Such properties (such as from flame weapon) are resolved independently, before the damage from the weapon itself is calculated.
There is a distinction between a damage bonus (on a weapon) and a damage increase effect (on a creature). Some spells and abilities (e.g. deafening clang) provide a damage bonus that remains with the targeted weapon (so swapping weapons means that the bonus is no longer used). Other spells and abilities (e.g. prayer) provide a damage increase to a creature that remains in effect regardless of the weapon being used. Distinguishing these two cases can be confusing at times, since both can be called "bonus damage", and many spells that target weapons allow the targeting of a creature with an implied targeting of that creature's main-hand weapon (or creature weapon).
Ranking[]
Each damage bonus amount has a "rank" (builder note: defined via iprp damagecost.2da). When two damage bonus properties do not stack, the one with the higher rank is used. Higher ranks generally correspond to higher average damage, so usually the property with the higher average damage is used, but the game does not guarantee this. In fact, there are two exceptions in that +2d4 (average +5) is ranked above +1d10 (average +5.5), and +2d12 (average +13) is ranked the same as +11.
The ranks of the various bonuses are the following.
| Rank | Amount | Rank | Amount | |
|---|---|---|---|---|
| 1 | +1 | 16 | +9 | |
| 2 | +2 | 17 | +2d8 | |
| 3 | +1d4 | 18 | +10 | |
| 4 | +3 | 19 | +2d10 | |
| 5 | +1d6 | 20 | +2d12 and +11 (tied rank) | |
| 6 | +4 | |||
| 7 | +1d8 | 21 | +12 | |
| 8 | +5 | 22 | +13 | |
| 9 | +1d10 | 23 | +14 | |
| 10 | +2d4 | 24 | +15 | |
| 11 | +6 | 25 | +16 | |
| 12 | +1d12 | 26 | +17 | |
| 13 | +7 | 27 | +18 | |
| 14 | +2d6 | 28 | +19 | |
| 15 | +8 | 29 | +20 |