Ballatar's Buckler (Patreon)

Armor (shield), rare (requires attunement)

This flat metal buckler has a bladed edge that encircles it. Three prongs support the blade, which can be separated from the face of the shield. While the blade is attached to the shield, you can use a reaction whenever a creature misses you with an attack while within 5 feet of you to make a single attack with the shield, treating it as if it were a scimitar with which you're proficient.

You can use an action while holding the shield to speak its command word, causing the blade to lift from the surface of the shield and rapidly spin. It then flies off the face of the shield in a direction of your choice. Choose a path for the spinning blade to move, which has a flying speed of 90 feet. The blade only moves in straight lines and can bounce from objects to change course. It costs the blade 10 feet of movement each time it strikes a target. Any creature in the path of the spinning blade must make a DC 15 Dexterity saving throw the first time it passes through the creature's space, taking slashing damage equal to 1d6 + your Strength or Dexterity modifier (your choice) on a failed save.

If the spinning blade flies back into your space, you can choose for it to reattach to the shield or catch it in your free hand; otherwise, it falls to the ground at the end of its movement. The blade can be reattached to the shield as an action. Until the blade is reattached, you can use it as a magic scimitar with the thrown property (30/90). You're proficient with this weapon. If the blade remains detached from the shield for 24 hours, it magically reappears on the shield if you're both on the same plane of existence.

 
Putting a new spin on a good offense is a good defense.