Magical Tattoos

Sometimes tattoos are more than just for show. As with any other magical item, tattoos can be infused with magics to generate all sorts of effects. They are at advantage because they cannot be stolen or disarmed, and destroying them generally means destroying the wearer. The tattoo will continue to function even if the body is damaged or defaced.
They also have some down sides. You can't easily share the item with a friend, and magical tattoos almost always require attunement. So you are limited in not only the number of magical tattoos you can have, but other magic items as well.
Best to make sure to have a magical tattoo applied by someone reputable as well. Failing to do so could result in painful and sometimes dangerous...complications.

Attunement

A magical tattoo, once created, is a specific kind of wondrous item which exists in a special magical needle called a stylus. You must apply the tattoo to attune to the item.
You apply the tattoo by spending a short rest holding the stylus to your skin where you want the tattoo to appear. When complete the stylus turns into the ink that becomes the tattoo which appears on the skin and to which you are now attuned.
If your attunement to the tattoo ends, the tattoo vanishes, and the stylus reappears beside you.

A Work of Art

What the tattoo looks like once applied depends on how it was created in the first place. The creator has a choice while crafting the magical stylus. The final product will contain the true art of a tattooist, or be customizable when the wearer attunes to the item.
The crafter can spend extra time and money on magical inks and materials to allow for the tattoo's appearance to be customizable whenever it is applied. This will result in an increase to the magical tattoo's cost of 20% or more, but no tattoo artist is required for the creation of such an item.
Alternatively, if a tattoo is properly crafted by a trained tattooist it can be drawn into the finally prepared stylus and will maintain the appearance created by the artist whenever applied. This is less expensive with no extra time and materials required. Once a physical tattoo is drawn into a stylus it is forever set as that magical tattoos appearance. Some feel this method yields a nicer result artistically.
The original tattoo could be created on and be drawn out of a living subject, or another suitable medium such as a hide. In fact, some sellers of magical tattoos may have a selection of tattoos on hides so that the buyer has a choice of what their tattoo will look like.
It is not common that the magic item crafter is also the skilled tattooist, but it has happened.

Rarity and Size

Different magical items come in different rarities, and how this plays out with magical tattoos is in determining how much space the tattoo typically takes up on the body.
There may be exceptions, but generally at common and uncommon rarity a magical tattoo is Small. Rare magical tattoos are Medium sized. Very rare magical tattoos are Large size, while Legendary ones are at least Large maybe larger.

Tattoo Examples

Gravesealer Tattoo

Wondrous item (tattoo), very-rare (requires attunement)
Zombie Bane. Your unarmed strikes deal an extra 1d4 radiant damage on a hit to undead creatures. Lich's Bane. When you hit any creature that has a phylactery with an unarmed strike they must make a Wisdom Save DC 16 or be destroyed. Once restored by their phylactery, the phylactery is also destroyed freeing all souls within it.

Greater Blade Pact Tattoo

Wondrous item (tattoo), rare (requires attunement by a warlock with the pact of the blade feature)
Beyond Limits. Your Pact of the Blade feature can now affect an artifact or a sentient weapon.

Life Bond Tattoo

Wondrous item (tattoo), uncommon (requires attunement)
Bound. A pair of tattoos which must be attuned together. Wearers can locate each other within 1,000ft.
'Til Death. While both wearers have more than 1 hit point, when one of the wearers would be reduced to 0 hit points they drop to 1 hit point instead. The other wearer receives the remaining damage which can't be reduced or prevented.

Mark of Elvenkind Tattoo

Wondrous item (tattoo), uncommon (requires attunement)
Fey Ancestry. You have advantage on saving throws against being charmed, and magic can’t put you to sleep.
Trance. You don’t need to sleep. Instead, you meditate deeply, remaining semiconscious, for 4 hours a day.

Shapechanger Tattoo

Wondrous item (tattoo), very-rare (requires attunement)
All Shapes and Sizes. As an action you can cast Alter Self, Enlarge/Reduce, or Polymorph on yourself. You can use this feature a number of times equal to your proficiency bonus, and regain all expended uses of it when you finish a long rest.
Fast Healing. As an action you cast Regenerate on yourself. This property can’t be used again until you finish a long rest.

Shielding Tattoo

Wondrous item (tattoo), uncommon (requires attunement)
Shield of Ink. When hit with an attack, you can use your reaction to add +2 to your AC, including against the triggering attack, until the start of your next turn. You can use this feature a number of times equal to your proficiency bonus, and regain all expended uses of it when you finish a long rest.

Syndetic Tattoo

Wondrous item (tattoo), uncommon (requires attunement)
Linking the Ink. While this tattoo is on your skin, you may attune up to two more magical tattoos without those counting against your attunement limit. If this tattoo becomes unattuned, those tattoos do as well. More than one Syndetic Tattoo can be attuned, but all tattoos still take up space.

Tiny Dancer Tattoo

Wondrous item (tattoo), uncommon (requires attunement)
Strut for the Hut. You may cast the spell Tiny Hut as a ritual, however you and all creatures within the dome must dance for the duration of the ritual while you sing the *Tiny Hut* song. This property can’t be used again until you finish a long rest.