Shield Bash

You may now use your shield to strike someone. Doing so deals damage equal to your strength modifier + proficiency bonus. Alternatively, you may choose to deal no damage instead stun your target. 

Shield bash requires you to make an unarmed attack with your shield. It requires an action to do, if you hit you may choose to deal damage or apply the stunned. 

Stunning - you may only use this feature once a round, on a successful hit the target becomes stunned, the target may make a Constitution saving throw; DC 8 + Strength Modifier + Proficiency Bonus, at the end of your turn to break the  stunned condition. If they fail they are  stunned till the end of their next turn. 

You may only use shield bash on a creature smaller than you and up to one size category larger.