4. When Snape killed Dumbledore. In probably the most shocking twist of the series, Snape cast the unforgivable Avada Kedavra curse on Hogwarts' beloved headmaster.
Out of desperation, Harry used it on Malfoy, as Malfoy was preparing to hit Harry with the Cruciatus curse. It apparently causes the caster's wand to act as a very long knife, thus resulting in the target getting huge cuts across their body.
Snape is a powerful wizard, and he is able to concentrate the necessary emotion to make the curse work. Perhaps emotions other than hate can power the killing curse. Snape undoubtedly sees the killing as an act of mercy towards Dumbledore, and indirectly, towards Draco.
Harry makes use of two of the Unforgivable Curses in the books. Dueling with Bellatrix Lestrange, he attempts the Cruciatus curse, with limited results; Bellatrix says that he has to really hate someone to make the Unforgivable Curses work properly, righteous indignation isn't enough.
'Is Draco alive? Is he in the castle? ' The whisper was barely audible; her lips were an inch from his ear, her head bent so low that her long hair shielded his face from the onlookers.
Rictusempra, the Perpetual Tickling charm, which apparently can be stopped by the Finite Incantatem charm, was used by Harry against Malfoy in the Duelling Club. The intent was, of course, to tickle Malfoy to the point that he would be unable to jinx Harry in return.
For using Dark Magic, albeit unknowingly, to cause serious harm, Snape sentences Harry to detention every Saturday until school ends. Snape dismisses Harry's protests, as does Professor McGonagall, who later informs Harry that he was lucky not to be expelled.
Cruciatus Curse ('Crucio')
Repeated use of the Cruciatus Curse is torture and Bellatrix Lestrange subjected the Longbottoms to such extremes of pain that it sent them insane.
Fantastic Beasts: The Secrets of Dumbledore confirmed that Credence Barebone/Aurelius Dumbledore (Ezra Miller) is the new Snape (Alan Rickman) through a very familiar line delivered by Aberforth Dumbledore (Richard Coyle). Fantastic Beasts 3 finally shed light on the Credence being Aurelius Dumbledore plot twist.
Vulnera Sanentur was the song-like incantation of a healing spell and counter-curse to the Sectumsempra Spell, which was invented by Severus Snape.
It was Snape who taught Harry his signature spell: Expelliarmus. During Duelling Club in Harry's second year, Snape used the Disarming Charm against Lockhart.
Avada Kedavra, also known as the Killing Curse, kills a person instantaneously and without injury. There is no countercurse for it, and only one person, Harry Potter, has ever survived it.
They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
After being on the run from a bunch of Snatchers, the gang were taken to Malfoy Manor, where Bellatrix proceeded to torture Hermione using the Cruciatus Curse.
Narcissa's greatest moment, however, was when she chose to betray Voldemort in Harry Potter and the Deathly Hallows. Not knowing whether Draco was alive or dead, Narcissa chose to lie to the Dark Lord rather than risk losing her son.
Following his failure to obtain an important prophecy at the end of Order of the Phoenix, Lucius is sent to Azkaban for a while until Voldemort breaks him out, although he doesn't play much of a role in the series going forward.
Why? The main reason is that Lucius failed to retrieve the prophecy from the Ministry of Magic, put that on top of all the other times Lucius failed, and it leads to him losing any clout he once carried within the Death Eaters. Lucius did that for his own benefit - he was trying to discredit Arthur weasley.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Expelliarmus did not beat Avada Kedavra.
Voldemort's wand (the elder wand) recognized its true master (that was Harry) and refused to attack it. In a way Voldemort killed himself as his own spell backfired on him.