The Marauder's Map is special in the mythos of the Harry Potter universe. With it, Harry can see the entire grounds of Hogwarts, including secret spots that most students (and probably some teachers) don't even know about.
In order to see the complete grounds though, Harry had to recite a spell that allowed him to discover these undisclosed parts of the map. That spell of course is, "I solemnly swear that I’m up to no good."
Tattoo enthusiasts have used this secret spell concept as an inspiration for Harry Potter-themed ink.
Blacklight tattoos are done with UV (or blacklight) ink, which means they are virtually invisible to the naked eye unless they are seen under a blacklight.
Tattoos like this can seem like magic, but they are quite common if you (like all things) go to an artist that knows what they are doing.
Since UV light tattoos are pretty much invisible under regular light, they take a much longer time to complete and should only be done by experienced artists. They should definitely have a blacklight handy as well so they can go over their work and make sure it has been done correctly.
If you're more interested in showing off your Harry Potter love to the world, and don't want it to be a secret, you might be inspired by these tattoos.
But if you're looking for a low-key solution to showing off your wizardry, you can't go wrong with these discreet ink jobs.