Gellert Grindelwald
Considered by many to be the darkest wizard of all time before Lord Voldemort, Grindelwald was defeated by Albus Dumbledore. As a student, Grindewald attended Durmstrang. While expelled from school, he met a young Albus Dumbledore in Godric's Hollow. There, the two became close friends and conspired to capture the Deathly Hallows. Their friendship fell apart when Albus Dumbledore accidentally killed his sister while in a dual with Grindlewald.
Grindlewald left Britian and then stole the Elder Wand. Killing many witches and wizards, Grindlewald rose to power, just to be defeated by Albus Dumbledore.