Long ago, Dumbledore and Grindelwald got matching tattoos, enchanting them so that, when touched they'd lead them to each other (like the Dark Mark). The idea was that, if ever either one needed the other, either because he'd found a Hallow or because he was in trouble. After their estrangement, neither one used it. Until one day, Dumbledore (not yet as powerful as he is at the time of the HP series) finds himself in dire straits, and uses the tattoo. To his surprise, Gellert comes to his aid.