Mighty bonus is an item property for bows. It allows the user of the weapon to add a damage bonus equal to his or her strength modifier or the mighty bonus, whichever is the lower. This bonus is similar to the damage bonus applied to melee weapons for having a strength greater than 11.
Note that the character sheet in NWN2 is bugged when it comes to bows. It will always will show a damage bonus as if wielding a two-handed weapon (Strength modifier x 1.5), even though the actual damage increase is first of all capped by the equipped bow's mighty bonus and secondly is applied as if wielding a one-handed weapon (Strength modifier x 1).
|Weapon||Strength||Mighty||Damage Shown||Actual Damage|
|Composite Longbow +1||14 (+2)||1||1d8+3||1d8+1|
|Composite Longbow +3||18 (+4)||3||1d8+6||1d8+3|
|Composite Longbow +5||18 (+4)||5||1d8+6||1d8+4|
When enchanting bows, keep in mind that mighty takes up an enchantment slot. So does weight reduction. So in the OC, where you have up to three max enchantment slots, a Shederran bow has one slot already taken up and a Duskwood bow has two. Meanwhile, there is no way to improve the mighty bonus or attack bonus of a bow via enchanting. So, for PC's with sufficient strength bonuses, the best bows (arguably) to enchant with additional properties are either a composite longbow +5 (if you can find one) or a Shederran longbow.
NWN & 3.5 Rules Comparison
In the Pen & Paper version of Dungeons and Dragons, a user of a bow with mighty bonus would receive an attack bonus penalty equal to the difference between the mighty bonus and the strength modifier, if he or she did not have a strength modifier equal to or greater than the mighty bonus.
This was not the case in NwN1 and possibly not the case on NwN2 either.