The “Crimes of Grindelwald” refers to the decades-long reign of terror orchestrated by the Dark Wizard Gellert Grindelwald across continental Europe, which culminated in his defeat in 1945. The ideological roots of his campaign began in the summer of 1899 in Godric's Hollow, where a young Grindelwald formed a close bond with Albus Dumbledore. Together, they developed a revolutionary plan to end the International Statute of Secrecy and establish a global wizarding order to rule over Muggles for what they termed “the Greater Good.” This alliance shattered following a three-way duel between Albus, his brother Aberforth Dumbledore, and Grindelwald, which tragically resulted in the death of their sister, Ariana Dumbledore. Following this incident, Grindelwald fled and began to amass power. A key step in his ascent was stealing the Elder Wand from the famed European wandmaker, Mykew Gregorovitch. With this incredibly powerful wand in his possession, Grindelwald began his campaign of conquest. He established a fearsome reputation, and his personal symbol—the mark of the Deathly Hallows—became synonymous with him and the Dark Arts. Viktor Krum noted that Grindelwald carved the symbol into a wall at the Durmstrang Institute and that his crimes included the murder of many witches and wizards, including Krum's own grandfather. Grindelwald built a fortress, Nurmengard, to imprison his enemies. The slogan from his youth, “For the Greater Good,” was chillingly carved over its entrance. While his power grew to dominate much of Europe, he notably never attempted to seize control of Britain, a decision later attributed to a lingering fear of or affection for Albus Dumbledore. The conflict reached its zenith in 1945 when Dumbledore, who had for years resisted confronting his former friend, finally engaged Grindelwald in a legendary duel. Dumbledore emerged victorious, ending Grindelwald's reign. Grindelwald was subsequently imprisoned in the highest cell of his own prison, Nurmengard, where he remained for over fifty years until he was sought out by Lord Voldemort in 1998. In a final act of what Dumbledore believed was remorse, Grindelwald lied to Voldemort about the Elder Wand's whereabouts and was killed for his defiance.
While the main events of Grindelwald's terror campaign occur long before Harry Potter's birth, his history is a cornerstone of the plot in Harry Potter and the Deathly Hallows.