Here's a wild understatement: Harry Potter fans are a devoted bunch. Just look to the countless tattoos dedicated to the series — one that we reported on was even in J.K. Rowling's handwriting — because, well, who wouldn't want memories of Harry, Ron, and Hermione on their bodies for life? The latest ink design to go viral might just be the most wizard-approved of them all. But, it also comes with a caveat.The tattoo shows several sets of black footprints — nothing too out of the ordinary there. But when a blacklight comes into the mix, things get trippy. "I solemnly swear that I am up to no good," the text reads — a nod to Marauder's Map, as the HP fan base obviously knows. The hidden tattoo has quickly become an internet hit, with 1.3 million views on Imgur . But, as Mic points out , the UV ink could potentially be hazardous."Many inks that glow in the dark contain phosphorus, a dangerous element that can be incredibly toxic to humans depending on where it's derived," Joel Schlessinger, MD, a dermatologic surgeon, told Mic. "Because tattoo ink is not regulated by the FDA, it can be difficult to know if the ink to be used contains even the smallest amount of phosphorus."New York City tattoo artist Scott Campbell of Saved Tattoo says he doesn't endorse UV ink, nor has he ever used it since UV-reactive chemicals don't tend to be safe for the skin. On the other sleeve, err, hand, artist JonBoy of NYC's West 4 Tattoo says he's had a UV-ink tattoo for years without any negative side effects. "I've been in the business for over 16 years. I remember when UV first hit the market. At the time, it was fairly new so people were a little iffy about how safe it was," he says. "I've had mine for years. I recommend it."