I think I have been watching Vikings for too long...
Shields can be used as an improvised weapon to deal 1d4 damage. No proficiency bonus to attack roll unless the wielder has the Shield Master or Shield Warrior Feats, the Gladiator Background or a Fighter Class (or any other background that may allow it, such as a Viking Warrior type background, talk to your DM). The shield is a special case for even though it is bulky and unwieldly, it can be used as a light weapon when two-weapon fighting with a shield (which means the primary weapon must be light as well). You do not gain defense bonus with the shield if you make an attack.
Now bladed shields or any such items will either gain a +1 damage or do 1d6. It is up to you DM if they will allow them. As for myself, well, specialized gladiators can have them.
Note, if a shield is magical, it does not add anything to any attacks it makes.
You know how to attack with your shield and still gain from it when using it to attack as a secondary weapon.
- Gains proficiency when attacking with the shield, if not already had.
- You can use two-weapon fighting you may use any one-handed melee weapon instead of just a light weapon. When you take the Attack action and attack with a melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different with the shield that you’re holding in the other hand. You may add your ability modifier to the damage of the bonus attack.
- Still gains a +1 Armor Class even when using the shield to attack.