Gellert Grindelwald was a Dark wizard, who was defeated by Albus Dumbledore.

From the Story

Harry Potter and the Deathly Hallows

Discovered in chapter 8, The Wedding

Viktor Krum tells Harry that the triangular symbol worn by Xenophilius Lovegood is Grindelwald's sign. According to Krum, Grindelwald killed many people, including Krum's grandfather. He was never powerful in Britain, as he feared Dumbledore. Grindelwald carved his symbol into a wall at Durmstrang when he was a pupil there.