The ousting of Severus Snape (also [improperly] referred to 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.
As the group got down, they were stopped by Headmaster Severus Snape and the two teachers engaged in a duel. McGonagall was joined by Filius Flitwick, Pomona Sprout and Horace Slughorn. Now greatly outnumbered, Snape fled into a nearby classroom and flew out of a window.
Search of Ravenclaw Tower
Harry and his friends entered the Room of Requirement, where he found many students (mostly former Dumbledore's Army members) hiding so as to avoid the Carrow siblings, two Death Eaters teaching at Hogwarts known for their brutality and cruel, sadistic punishments on wrongdoing students. Believing the Horcrux to be a Ravenclaw-related object, and knowing that the only object known to have belonged to Ravenclaw was the Diadem of Ravenclaw, Harry Potter (accompanied by Luna Lovegood) decided to go to Ravenclaw Tower to get an idea of what the diadem looked like, as there was a model of it on a statue in the common room. They entered the common room, but Alecto Carrow caught them and pressed her Dark Mark with her finger, thus sending the warning to Lord Voldemort and the other Death Eaters that Harry Potter was in the Castle.
Skirmish in the Common Room
- Harry Potter: "Time's running out, Voldemort's getting nearer, Professor, I'm acting on Dumbledore's orders, I must find what he wanted me to find! But we've got to get the students out while I'm searching the castle — It's me Voldemort wants, but he won't care about killing a few more or less, not now —"
- Minerva McGonagall: "You're acting on Dumbledore's orders? We shall secure the school against He-Who-Must-Not-Be-Named while you search for this — this object."
- — Harry Potter urges Minerva McGonagall to secure the Castle against the Death Eaters[src]
In an instant, Luna Lovegood shot a Stunning Spell at Alecto, who collapsed. Amycus Carrow then went up to the Tower, and tried to break in because he did not know the answer to the door knocker's question. He was joined by Minerva McGonagall, who he ordered to open the door for him. Upon discovering his sister unconscious on the floor and no sign of Harry Potter (he and Luna were under the Invisibility Cloak), Amycus panicked as he thought of Voldemort's fury at it being a false alarm.
Amycus then came up with a plan to blame Alecto's summoning of Voldemort to the castle on the Ravenclaw students, and McGonagall flatly told him blaming the two Death Eaters' failures on the students of Hogwarts was out of the question; she refused to allow him to lie and thus have the children punished by Voldemort. Amycus then spat on McGonagall's face, and in fury Harry Potter pulled the Cloak off up himself and cast the Cruciatus Curse on him, smashing the Death Eater into a bookcase.
McGonagall was shocked at the sudden appearance of both Harry and Luna, and he informed her that Lord Voldemort was on the way and that the Death Eaters would be soon attempting to break into the Castle. As Amycus rose grogily, McGonagall cast the Imperius Curse on him and made him hand both his and his sister's wand to her and then lie down next to his Stunned sister. McGonagall then conjured a length of shimmering silver rope that bound them tightly together and rose to the ceiling, leaving the two siblings dangling in the air.
Knowing that there was not much time, Harry told McGonagall that he was acting on Dumbledore's orders, which caused McGonagall to agree to secure the school from Voldemort while Harry looked for the Horcrux. Harry informed her that they could begin evacuating students through the passage going from Hogwarts to the Hog's Head Inn in Hogsmeade. She left the tower with Harry and Luna (the latter two under the Invisibility Cloak) in order to warn the Heads of House about setting up magical defences around the grounds.
- Severus Snape: "I wonder what could have brought you out of our bed at this late hour?"
- Minerva McGonagall: "I thought I heard a disturbance."
- Severus Snape: "Really? But all seems calm. Have you seen Harry Potter, Minerva? Because if you have. I must insist —"
- — A brief exchange between Severus Snape and Minerva McGonagall, instants prior to the duel[src]
As McGonagall marched toward the door, she raised her wand and cast three cat Patronuses, which ran ahead to alert the other Heads of House. They had descended two floors from Ravenclaw Tower when Harry noticed another quiet set of footsteps. McGonagall noticed them too, and she demanded the newcomer to announce his presence, and Severus Snape, the Headmaster, emerged from behind a suit of armour. Seeing only McGonagall, as Harry and Luna were hidden beneath the Invisibility Cloak, Snape asked her what had happened to the Carrows, and told her that he was under the impression that Alecto had apprehended an intruder.
Snape questioned McGonagall as to why she was patrolling the corridors at that hour, and then discreetly used Legilimency on the Professor. As he asked McGonagall if she had seen Harry Potter, McGonagall quickly slashed her wand through the air, but Snape cast a Shield Charm that threw McGonagall off balance. She then brandished her wand to a torch on the wall, making it fly off its bracket. The flames became a ring of fire that filled the corridor and flew like a lasso at Snape. Snape then turned the descending flames into a great black snake that McGonagall blasted to smoke and turned into a swarm of daggers, which she directed towards him. At this point, McGonagall begins to gain the upper hand as it becomes clear he cannot further transfigure or counter her attack, so the only way he avoids it is by taking refuge behind a suit of armour that he forces in front of him. 
At this point, Filius Flitwick, Pomona Sprout, and Horace Slughorn sprinted up the corridor toward them in their nightclothes. With a squeal, Flitwick raised his wand bewitching the suit of armour to attack Snape by crushing him. Snape, now greatly outnumbered, sent the suit of armour flying back against his attackers, and it smashed into the wall and shattered. Snape was then chased by McGonagall, Sprout, and Flitwick into a nearby classroom, jumped through a window and flew away, a skill he presumably learnt from Lord Voldemort. Minerva contemptuously called after Snape a "coward".
- "He-Who-Must-Not-Be-Named is coming. Potter has work to do in the castle on Dumbledore's orders. we need to put in place every protection of which we are capable while Potter does what he needs to do."
- —McGonagall talking to the other Heads of House[src]
Harry and Luna (and Slughorn, who had fallen behind) followed the teachers into the classroom. Urgently, Harry told McGongall that they needed to hurry, and McGonagall told the other teachers that they needed to protect the Castle from the Death Eaters while Harry searches for the Horcrux.
The teachers arranged so that they met in twenty minutes at the Great Hall in order to evacuate underage students. Flitwick started casting protective enchantments around the Castle, while Harry questioned him about possible locations of the diadem.
Meanwhile, Pomona Sprout departed for the Herbology greenhouses to retrieve dangerous plants (Venomous Tentacula, Devil's Snare, and Snargaluff pods) to hold off the Death Eaters while Slughorn went to wake his Slytherins in the dungeons. Argus Filch then arrived, complaining about students being out of bed, and McGonagall told him to get Peeves, the resident poltergeist, to join the battle that would soon ensue. She then bewitched the school's suits of armour to man the boundaries and protect the school, telling Harry and Luna to return to the Room of Requirement and bring the inhabitants to the Great Hall as well while she roused the Gryffindors and brought them there as well to decide who would be evacuated and who would stay and fight.
Behind the scenes
- "The Sacking of Severus Snape" is the title of chapter 30 of the seventh book, in which the duel takes place. However, this name is technically inaccurate, as "sacking" or "dismissal" refers to the termination of employment by the employer, against the will of the employee. Neither of the fighters who banished Snape from Hogwarts were his employers, more so, they were his subordinates. However, it could have been named this because Minerva, Filius, Pomona and Horace no longer respected Snape as their Head nor as their equal.
- In the second part of the film adaptation of Harry Potter and the Deathly Hallows, this duel has some noticeable differences:
- Unlike in the novel, Severus Snape hears of Harry Potter's sighting in Hogsmeade and summons all students to a meeting at the Great Hall. Harry Potter, who was in the Room of Requirement at the time of the summons, dresses in spare Gryffindor robes and goes down to the Great Hall blended in with the other students. Alecto and Amycus Carrow are also present, as well as all members of staff apart from Argus Filch and Sybill Trelawney. After Snape threatens to severely punish every student who fails to tell him about Harry's whereabouts, Harry reveals himself and members of the Order of the Phoenix back him up. Minerva McGonagall, however, steps up and singlehandedly fends Snape off using a fiery spell. An intense duel breaks out, in which the Carrows are left knocked out as Snape deflects McGonagall spells and they are hurled to the floor. Before McGonagall can attack again, Snape Apparates backwards across the hall, smashes through a window and escapes.
- It is possible that in the film, Snape deliberately deflects McGonagall's spells against the Carrows given his allegiance, but this is not confirmed.
- In the film adaptation, Filius Flitwick, Pomona Sprout and Horace Slughorn do not take part in the duel, but all are present in the room and pointing their wands at Snape ready to help McGonagall.
- In the film adaptation all students witness the duel, unlike the book in which no one seems to have noticed that Snape fled from the Castle until McGonagall tells the student body "he has, to use the common phrase, done a bunk".
- In the film Snape does not attack back against McGonagall but instead blocks all of her attacks before fleeing.
- The spell that McGonagall used to turn Snape's transfigured snake into a cloud of smoke may have been Vipera Evanesca.
- During one of the takes while filming this scene, Alan Rickman, while gesturing with his prop wand, hit his hand against the wires of a camera, dropping his wand and exclaiming "Ouch."
- In a deleted scene, before Harry entered the Great Hall with a Gryffindor uniform he walked down the steps of the Grand Staircase to hold hands with Ginny down with the Gryffindors.
- In the LEGO game both Harry and McGonagall duels Snape. This is done to make it more co-op friendly.
- In the PSP version of LEGO Harry Potter: Years 5-7 both the novel's battle and the film's battle is used:
- It takes place in the Great Hall.
- Filius Flitwick and Pomona Sprout help Minerva McGonagall.
- In the video game adaption a battle takes place between Harry Potter and Alecto and Amycus Carrow before McGonagall faces Snape.
- Harry Potter and the Deathly Hallows (First appearance)
- Harry Potter and the Deathly Hallows: Part 2
- Harry Potter and the Deathly Hallows: Part 2 (video game)
- LEGO Harry Potter: Years 5-7
- Harry Potter: The Character Vault (Mentioned only)