Shield Bashing (5e Variant Rule)
From D&D Wiki
Creatures that are proficient in shields can use a shield as an improvised melee weapon with the Light quality that deals 1d6 + Strength modifier bludgeoning damage. A creature is not considered proficient when attacking with a shield in this way.
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.
Characters who gain the Fighting Style class feature in a class that has proficiency in shields may select the Shield Bashing fighting style.
Shield Bashing (Fighting Style)
Whenever you succeed in shoving a creature while wearing a shield, the creature also takes damage as if you had hit it with an attack using a shield.