Gellert Grindelwald

Grindelwald was a Dark wizard defeated by Professor Dumbledore in 1945. His name is mentioned on Dumbledore's Chocolate Frog card.

Little information about Grindelwald appears in the novel, but the fact that he has a German-sounding name and was defeated by an English wizard in 1945 led many fans to believe he was somehow related to the German Nazis. J.K. Rowling confirmed this in an interview, where she said that the Wizarding world and the Muggle world influence each other enough that events in the two worlds often contribute to each other.

Seeing the date he died and the date of Tom Riddle's seventh year, it is possible that he was Voldemort's master.