The Battle of the Lightning-Struck Tower

"A jet of green light shot from the end of Snape's wand and hit Dumbledore squarely in the chest... he [Harry] was forced to watch as Dumbledore was blasted into the air... he seemed to hang suspended beneath the shining skull, and then he fell slowly backward, like a great rag doll, over the battlements and out of sight."

- Description of Dumbledore's death in the Astronomy Tower

Background
There were numerous casualties of war in 1996. When Minister Cornelius Fudge refused to surrender, Voldemort destroyed a section of a British bridge, drowning many Muggles. Florean Fortescue and Ollivander both went missing. Ollivander's shop was emptied, but there was no sign of struggle. Florean Fortescue's icecream parlour showed evidence of a struggle or fight, but no sign of him was ever found. A Man named Octavius Pepper disappeared in the early spring. Igor Karkaroff was found dead under a Dark Mark after he deserted Voldemort. Hannah Abbott's mother, Order of the Phoenix member Emmeline Vance, and Head of Magical Law Enforcement Amelia Bones were murdered by unknown assailants. These events allowed Head Ministry Auror Rufus Scrimgeour to replace Fudge as the new Minister for Magic.

During this time, Voldemort also gathered the strengths of the Giants, Dementors, and Werewolves to his cause.

Course of battle
At the close of the Hogwarts school year in June 1997, Albus Dumbledore was slain by Severus Snape's Killing Curse in the Hogwarts Astronomy Tower. He had been previously sickened to the brink of exhaustion by an unknown poison. After convincing Draco Malfoy to forego his plans to murder him, Severus Snape and four other Death Eaters came to the scene. Snape took over the task of killing Dumbledore.

Among the Death Eaters present were siblings Alecto and Amycus Carrow, Yaxley, Thorfinn Rowle and Gibbon, along with werewolf Fenrir Greyback. On the side of Hogwarts were siblings Ginny, Ron, and Bill Weasley, Hermione Granger, Luna Lovegood, Neville Longbottom, Remus Lupin, Nymphadora Tonks, and Minerva McGonagall. Bill Weasley was attacked by werewolf Fenrir Greyback and received scars on his face. Because Greyback was not fully transformed at the time of the attack, Bill sustained minimal lycanthropic symptoms. On the Death Eaters' side, the only death was that of Gibbon, killed by a stray Killing Curse from a fellow Dark Wizard.

The Death Eaters were forced to retreat. Much of the luck of the Hogwarts' defenders can be atributed to Felix Felicis, the potion of good fortune, which was used by the students who were present.