Gellert Grindelwald

Gellert Grindelwald is a Dark wizard who, in a list of Most Dangerous Dark Wizards of All Time, would be second only to Lord Voldemort, according to Rita Skeeter's book The Life and Lies of Albus Dumbledore. Grindelwald's name is fairly well known in the wizarding world. His name is first mentioned on Albus Dumbledore's Chocolate Frog card, which notes that Dumbledore defeated Grindelwald in 1945.
Grindelwald attended the wizarding school Durmstrang, from which he was expelled in his sixth year for his dangerous and evil experiments that nearly resulted in the deaths of some of his fellow students. He left the symbol of the Deathly Hallows (a circle with a vertical line in the centre enclosed in a triangle) on one of the walls in Durmstrang before departing. After Durmstrang, he went to live with his great-aunt Bathilda Bagshot in Godric's Hollow, where he met Dumbledore as a young man. In Deathly Hallows, it was revealed that Grindelwald had gone to Godric's Hollow to investigate the grave of the Peverells (the original owners of the Hallows), but struck up a close friendship with Dumbledore, and the two planned to establish a new world order, where wizards would rule over Muggles "for the greater good". They also planned to work together in their quest for the Hallows. It was revealed by Rowling in an interview that Dumbledore was once in love with Grindelwald, but did not say whether those feelings were returned.[5][6]
However, Aberforth Dumbledore, Albus's younger brother, argued against these plans, because he feared their grand ambitions would leave his disabled, traumatised sister, Ariana, abandoned. Grindelwald accused him of being blind and argued that Ariana would not have to be hidden once wizards ruled the world. The argument culminated in a three-way battle between Albus, Aberforth, and Grindelwald. Ariana was inadvertently killed by one of them. Grindelwald fled, fearing retribution. Henceforth, Albus Dumbledore ended his friendship with him.
Grindelwald successfully obtained one of the Deathly Hallows, the Elder Wand. He became master of the legendary wand by stealing it from the previous owner, the wand-maker Gregorovitch. Gaining the Elder Wand's immense power, he subsequently committed many terrible acts, although these seem to have been restricted to Central and Eastern Europe. It is revealed that Grindelwald's actions have caused many deaths that have greatly affected the students of Durmstrang, including Viktor Krum, whose grandfather was murdered by Grindelwald. As a result, Grindelwald and anything associated with him (including the symbol of the Deathly Hallows) is almost universally hated at that school.
After Grindelwald's rise to power, Dumbledore delayed meeting him again for several years due to his fear of being confronted with his sister's death and the fact that he himself might have been the one who accidentally killed her. Both wizards were highly intelligent and skilled in battle, and those who witnessed the battle say that no other wizarding duel ever matched it. Grindelwald, who at the time possessed the supposedly unbeatable Elder Wand, lost to Dumbledore. Since Dumbledore won the duel over Grindelwald, the Elder Wand transferred its allegiance to Dumbledore. Dumbledore was now its rightful master.
After Dumbledore triumphed over Grindelwald, the defeated dark wizard was imprisoned in the top-most cell of Nurmengard, a prison Grindelwald himself had built for his opponents. He remained there until the events of Deathly Hallows when Voldemort arrives, seeking the Elder Wand. Grindelwald, showing no fear of Voldemort and welcoming death, tells him that he never owned the wand, and Voldemort kills him in rage. In the chapter King's Cross, Harry suggests to Dumbledore that Grindelwald lied to Voldemort in order to prevent him from breaking into Dumbledore's tomb, where he knew the Elder Wand lay. Dumbledore suggests that in his later years Grindelwald had felt remorse for his evil actions.