Attack roll

To attack an opponent, roll a d20 and add your character's attack bonus. If the result equals or exceeds the opponent's Armor Class (AC), the attack succeeds. On a successful attack, roll the dice indicated for the weapon you used to determine how much damage your attack deals. Damage reduces hit points (hp). When all of a character's hit points are gone, the character falls unconscious and is dying. A critical hit deals more damage. If you roll a natural 20 on an attack roll, you threaten a critical hit. Roll again to confirm it If the second attack roll is successful, then the critical hit is confirmed and you deal more damage.

External resources

 * NWNWiki:Attack roll