Mace of Smiting

You gain a +1 bonus to attack and damage rolls made with this magic weapon. The bonus increases to +3 when you use the mace to attack a construct.

When you roll a 20 on an attack roll made with this weapon, the target takes an extra 7 bludgeoning damage, or an extra 14 bludgeoning damage if it's a construct. If a construct has 25 hit points or fewer after taking this damage, it is destroyed.

Note: According to the SRD, it is an extra 2d6 and 4d6 bludgeoning damage, although this is incorrect.