Gellert Grindelwald

"Kill me, then! You will not win, you cannot win! That wand will never, ever be yours!"

- Grindelwald taunting Voldemort, shortly before his death

Gellert Grindelwald is a minor villain, yet a very important background character from the Harry Potter series of books and their movie adaptations. He is considered by many to have been one of the most powerful Dark Wizards in history, only second to Lord Voldemort. He attempted to take over Europe during the Second World War, but was defeated by the protagonists' mentor Albus Dumbledore in 1945 and imprisoned until his death to the Dark Lord.

History
Grindelwald was schooled at Durmstrang Institute, a reputed scool of wizardcraft in Central Europe, rather infamous for its tolerance towards the Dark Arts, but was expelled at sixteen years old due to his unealthy experiments. He then took up a friendship with Albus Dumbledore (who wound up falling in love with him) when he lived in Godric's Hollow for a summer with his great-aunt, Bathilda Bagshot, following his expulsion.

The two made plans to find the Deathly Hallows and create a world in which Muggles would be subservient to wizards and witches. This partnership fell apart after the two were involved in a three-way duel with Aberforth Dumbledore that resulted in Ariana Dumbledore's death. Grindelwald left Britain and soon stole the Elder Wand, an extremely powerful wand which vastly magnified its wielder's might, building an army and rising to power in continental Europe. During his reign of terror, he murdered many wizards and Muggles.

In 1945, at the height of his power, his former friend Dumbledore confronted him due to the cries of the public and defeated him in what became known as a legendary duel. Grindelwald was subsequently imprisoned in his own prison, Nurmengard, on 1945, where he remained for the rest of his life.

Death
When Voldemort was in search of the Elder Wand, he traveled to Nurmengard and questioned Grindelwald, who taunted Voldemort by his true name and refused to give him any information about, which Harry believed was because Grindelwald did not want Dumbledore's tomb desecrated as Grindelwald may have still considered Dumbledore as a friend and may have been remorseful about his actions. Angered by Grindelwald mocking him, Voldemort killed Grindelwald with the Killing Curse.

Trivia

 * He is played by Jamie Campbell Bower when he is younger and Michael Byrne as an older man.
 * In the film Harry Potter and the Deathly Hallows Part One, Voldemort doesn't kill Grindelwald, unlike the book, which says how Voldemort killed Grindelwald out of rage.