The death of Albus Dumbledore was a pivotal event in the Second Wizarding War, occurring on an evening in late June of 1997. Dumbledore, the Headmaster of Hogwarts School of Witchcraft and Wizardry and the leader of the Order of the Phoenix, was killed by Severus Snape atop the Astronomy Tower. The act was performed with the Killing Curse, Avada Kedavra. While witnessed by a hidden Harry Potter and members of the Death Eaters, the death was not a simple act of betrayal. It was, in fact, a pre-arranged mercy killing planned by Dumbledore himself. The event sent shockwaves through the wizarding world, marking the end of an era and forcing Harry Potter to proceed on his mission to destroy Voldemort's Horcruxes without his mentor's direct guidance.
The plan for Dumbledore's death was conceived after he located and destroyed one of Voldemort's Horcruxes, Marvolo Gaunt's Ring, during the summer of 1996. The ring was protected by a powerful and deadly curse. While Dumbledore successfully destroyed the Horcrux with the Sword of Godric Gryffindor, the curse afflicted his right hand, causing it to become blackened and dead-looking.
The plan was set in motion on the night Dumbledore took Harry Potter to a sea cave to retrieve another suspected Horcrux, the Locket of Salazar Slytherin.
Dumbledore's death had immediate and far-reaching consequences for the entire wizarding world and the war against Voldemort.