Gellert Grindelwald is a Dark wizard who, in a list of "Most Dangerous Dark Wizards of All Time", would be second only to Voldemort, according to Rita Skeeter's book The Life and Lies of Albus Dumbledore. His name is first mentioned on Dumbledore's Chocolate Frog card, which notes that Dumbledore defeated Grindelwald in 1945.
Grindelwald attended the wizarding school Durmstrang, from which he was expelled at age sixteen 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 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 the young Dumbledore. In Deathly Hallows, it was revealed that Grindelwald had gone to Godric's Hollow to investigate the grave of the Peverell brothers (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" (the phrase later became Grindelwald's excuse for the atrocities he committed in his reign of terror). They also planned to work together in their quest for the Hallows. Later Rowling claimed at a Q&A session that she implied Dumbledore being once in love with Grindelwald, and thus he has 'lost his moral compass', but she did not say whether those feelings were returned.
However, Aberforth Dumbledore, Albus' younger brother, argued against these plans, because he feared their grand ambitions would leave his disabled, traumatised sister, Ariana, abandoned. Later the argument culminated in a three-way duel among Albus, Aberforth, and Grindelwald. Ariana was inadvertently killed by one of them. Grindelwald fled, fearing retribution and henceforth, Albus ended his friendship with him. Grindelwald successfully became master of one of the Deathly Hallows, the Elder 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. 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.
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 when their battle eventually occurred, those who witnessed it later said 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. After Dumbledore triumphed over Grindelwald, the defeated dark wizard was imprisoned in the top-most cell of Nurmengard. He remained there, growing emaciated and toothless through the ravages of the prison, 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.
In Harry Potter and the Deathly Hallows – Part 1, young Grindelwald is portrayed by Jamie Campbell Bower. The older Grindelwald is played by Michael Byrne. In the film, unlike in the book, Grindelwald reveals to Voldemort the whereabouts of the Elder Wand, and thus is not killed.