Age of Heroes
Though shields are designed to provide defense against blows, you can also use them offensively, slamming them into opponents to cause additional damage. You can make a shield bash attack as an attack action or as a free action immediately following a melee attack. To make a shield bash attack, you proceed as if you were making an ordinary melee attack but use your Shield Defense bonus instead of your base attack bonus; the roll is thus 1d20 plus your Shield Defense bonus plus your Strength modifier. (You do not add the shield’s Coverage bonus to the attack roll.) If your character has no shield defense bonus, then you simply roll 1d20 plus your Strength modifier. If you make a shield bash attack, you use up one of your Shield Defense uses for the round. If you have already used up your Shield Defense checks, such as by deflecting a blow reactively before your turn, then you cannot make a shield bash.
Characters with the Improved Shield Bash feat may still make their normal number of Shield Defense checks after they have made a shield bash attack, albeit at a –4 penalty to the Shield Defense roll.