EffectDamageIncrease() NWScript command creates a damage increase effect, an effect that will increase the damage inflicted in combat by whatever it is applied to. This effect can only be meaningfully applied to creatures, and it should be applied temporarily or permanently (not instantly).
effect EffectDamageIncrease (int nBonus, int nDamageType=DAMAGE_TYPE_MAGICAL
Always use a DAMAGE_BONUS_* constant for the first parameter; anything else is likely to result in unintended behavior. (For example, using the number 6 will not result in 6 bonus damage.)
An invalid damage type (such as DAMAGE_TYPE_BASE_WEAPON) supplied as the second parameter defaults to magical damage.