The ousting of Severus Snape (also [improperly] referred as The Sacking of Severus Snape) was a duel between Severus Snape, then Headmaster of Hogwarts School of Witchcraft and Wizardry, and the Hogwarts Heads of Houses. Having defeated Alecto and Amycus Carrow on the Ravenclaw Tower, Harry Potter and Luna Lovegood (both under the Invisibility Cloak) along with Minerva McGonagall went down to warn the Heads of House that Lord Voldemort and his Death Eaters were about to siege the Castle in an attempt to get Harry Potter.