Gellert Grindelwald is first mentioned in Harry Potter & The Sorcerer's (Philosopher's) Stone on the back of Albus Dumbledore's wizard card. The card reads, Considered by many the greatest wizard of modern times, Dumbledore is particularly famous for his defeat of the dark wizard Grindelwald in 1945... . Nothing more is revealed about Grindelwald until the seventh and final book, Harry Potter & The Deathly Hallows, where it is revealed that Dumbledore and Grindelwald were friends in their teenage years. It is revealed that Grindelwald and Dumbledore were both exceptionally bright and talented but Grindelwald was expelled from Durmstrang in his third year for practicing the Dark Arts.After J.K Rowling announced that Dumbledore was gay, some have speculated that the relationship between Dumbeldore and Grindelwald may have been romantic. However, Rowling confirmed that, though Dumbledore may have developed feelings for Grindelwald, the feelings were not mutual.In the chapter The Missing Mirror of Harry Potter & the Deathly Hallows, much more information is revealed about Grindelwald. Aberforth, Albus Dumbledore's brother, confronted Albus and Grindelwald in their teenage years at Godric's Hollow and in response, Grindelwald ridiculed him and proceeded to use the Cruciatus curse on him. This escalated into a duel with Albus, Grindelwald, and Aberforth all involved. Many years later, in 'Harry Potter & the Deathly Hallows', Aberforth says that his sister, Ariana, saw them fighting and wanted to help them. When the battle ended, Ariana was dead. Aberforth says, I think she wanted to help, but she didn't really know what she was doing, and I don't know which of us did it, it could have been any of us - and she was dead. Ariana's ambiguous death would later be the cause of the tensions between Albus and Aberforth for the rest of their lives.After the duel in Godric's Hollow, Grindelwald and Dumbledore had become enemies, though Albus still felt responsible for Ariana's death. After Grindelwald's departure from Godric's Hollow, details are ambiguous about his life.However, it is known that Grindelwald eventually becomes considered to be the second most powerful Dark wizard of all time (The first being Voldemort). Albus Dumbledore and Gellert Grindelwald do not meet each other again until the famous duel between the two in 1945. Dumbledore defeats Grindelwald and Grindelwald is sent to Nurmengard where he remains for the rest of his life.In the book, it is mentioned that there are rumors that Grindelwald eventually felt remorse for his actions. Grindelwald is finally murdered by Voldemort in his prison cell during the events of 'Harry Potter and the Deathly Hallows'. Voldemort comes for information about the Elder Wand. Grindelwald laughs and says that he welcomes death. In Voldemort's rage, he kills Grindelwald with the Avada Kedavra curse.Other info:Grindelwald's sign is identical to the sign of the Deathly Hallows. An equilateral triangle with a line from its vertex to the midpoint of the opposite side with an inscribed circle in the middle of the triangle.