Shield Bash Variant Rule

Creatures that are proficient in shields can use a shield as an improvised melee weapon with the Light quality that deals 1d6 bludgeoning damage. A creature is not considered proficient when attacking with a shield in this way, unless they are proficient with improvised weapons. 

The type of damage dealt by shields can be changed if the shield is modified. For example, a spiked shield deals piercing damage, or a shield with sharpened edges can deal slashing damage. Buying a shield with such a modification or modifying an existing shield usually costs an additional 15 gp.