Gellert Grindelwald
An infamous dark wizard who was defeated by Dumbledore in an epic duel. Grindelwald believed in the superiority of wizards over all other creatures and promoted a system of tyranny based on the slogan "For the Greater Good." In the final book of the series, Harry Potter and the Deathly Hallows, it is revealed that Dumbledore and Grindelwald were not always enemies.