Source: Tasha's Cauldron of Everything

Wondrous item (tattoo), rarity varies (requires attunement)

Tattoo Attunement. To attune to this item, you hold the needle to your skin where you want the tattoo to appear, pressing the needle there throughout the attunement process. When the attunement is complete, the needle turns into the ink that becomes the tattoo, which appears on the skin.

If your attunement to the tattoo ends, the tattoo vanishes, and the needle reappears in your space.

Protection. While you aren't wearing armor, the tattoo grants you an Armor Class depending on the tattoo's rarity, as shown below. You can use a shield and still gain this benefit.

Rarity AC
Uncommon 12 + your Dexterity modifier
Rare 15 + your Dexterity modifier (maximum of +2)
Very Rare 18