Weakness is an entity value that increases the dice level of incoming damage, usually of a specific damage type. When an entity is subject to damage dice of a type for which it has Weakness, the damage dice level is increased by an amount equivalent to the corresponding Weakness value.
For example, if an entity with Fire Weakness 2 is subject to 2d8 Fire damage, the roll is modified to 2d12 instead.
Weakness applies to any sources of damage, not just those in a formal Damage entry, and it applies separately to base and enhancement dice for any such roll.
Entities may have base Weakness values to various damage types. Those base values can be replaced with new base values from Gear and other effects, with the last effect overriding all others. Text that performs this operation will have the Weakness amount written after the Weakness type, such as “the target gains Fire Weakness 2” or similar.
Weakness values may also be modified up or down from their current value. When this happens, the modifying number is written before the Weakness type and preceded by either a + (for increase) or - (for decrease). Example text might read “the target gains +1 Fire Weakness” or similar. In this case, you take the target's current Fire Weakness value and add 1 to it.
Weakness values cannot be reduced below 0 by effects that decrease Weaknesses. For instance, an entity with Fire Weakness 1 that receives a -2 Fire Weakness effect would have an effective Fire Weakness 0. However, if the entity received another +1 Fire Weakness effect during the same duration of the previous effect, they would still have the same result of Fire Weakness 0, since the full -2 modifier is still being applied.
The opposite of Weakness is Resistance. An entity that has both Resistance and Weakness should only use the larger of the two, but subtract the difference. For example, an entity with Fire Resistance 2 and Fire Weakness 1 would apply a result of Fire Resistance 1. An entity with the same value for both Resistance and Weakness would thus have them cancel out and apply no modifiers to the corresponding damage type.