This tattoo depicts protective imagery and uses ink that resembles liquid metal. As shown below, the tattoo grants you an Armor Class depending on the tattoo’s rarity while you aren't wearing armor. You can use a shield and still gain this benefit.
Rarity AC
Uncommon 12 + your Dexterity modifier (half limb covered)
Rare 15 + your Dexterity modifier (maximum of +2) (one limb covered)
Very Rare 18 Tattoo Attunement (two limbs or chest or upper back covered)
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 you have multiple magic tattoos, they count as a single magic item concerning the number of magic items you can attune to. If your attunement to the tattoo ends, the tattoo vanishes, and the needle reappears in the closest unoccupied space to you.
* - (needle)