There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
Although Harry was justifiably defending himself against Malfoy, who was about to cast the Cruciatus Curse, using an unknown spell was dangerous and reckless. However, despite this grievous action that could have permanently expelled him from Hogwarts, Snape only assigns Harry detention.
Draco Malfoy (Cruciatus and Imperius Curses)
He's never seen using the Killing Curse, but under strict orders from Voldemort, he's known to have used the other two.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
And the third, the Killing Curse, is named by Hermione; Moody mentions that it can't be blocked, and only one person is known to have ever survived it: Harry Potter. Harry makes use of two of the Unforgivable Curses in the books.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
1. Avada Kedavra. No prizes for guessing that the deadliest of them all would be this one. Avada Kedavra, or the Killing curse, is another Unforgivable curse, the most dangerous one ever.
Because the killing curse can be blocked. In the books, Dumbledore blocks it... with Fawkes. Because when you intentionally kill someone, you destroy your own soul, then again Dumbledore is in King's Cross station just like fetus Voldemort, so maybe he's there because he actually did kill his sister.
None of the good guys dare use the Killing Curse throughout the events of the movies and books, no matter how high the stakes and how difficult things are. But Ron Weasley is the exception, giving it a go in the Deathly Hallows: Part 2 movie when himself and Hermione look destined to die via Nagini.
It is seen that when Nagini was going to kill Ron and Hermione, Ron used a curse with a green flash.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
Snape used this spell on George Weasley in Harry Potter and the Deathly Hallows, cursing his ear off. Mrs. Weasley remarks that as his ear was severed by Dark magic, it will be impossible to grow it back. Mentioning that it was Snape who did it, Lupin remarks that "Sectumsempra was always a speciality of Snape's."
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.
Sectumsempra is a curse invented by Professor Severus Snape, during his childhood, when he was known as "The Half-Blood Prince". He created it with the intention of using it against his enemies, and it soon became one of his specialties.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
Because he didn't believe disarming him would be enough. After Dumbledore was dead, Voldemort would have been told everything. Including the fact that Draco disarmed Dumbledore. But all he cared about was who KILLED Dumbledore.
Firestorm. Firestorm was a spell that created a large ring of fire around the caster's wand. This spell is one of the last and most powerful spells that Albus Dumbledore cast using the elder wand before his death.
Avada Kedavra
Topping the list for the most powerful spells in the Harry Potter universe is no other than Avada Kedavra, Voldemort's favorite to instill his will and fear onto others. The spell itself is a play on Abra Kadabra and is easy to cast by just about anyone in the Wizarding World.
3 Albus Dumbledore: Firestorm.
The three Unforgivable Curses in Harry Potter are Crucio, Imperio, and Avada Kedavra, and all three are available to Hogwarts Legacy players. Crucio subjects the victim to extreme torture, while Imperio puts them under the control of the spellcaster.
Confringo. Commonly known as the Blasting Curse, Confringo causes the target to explode. To make it even more dangerous, it produces flames that ignite upon contact with the target. If Confringo fails to kill the target immediately, they will instead suffer a slow and painful death.
In an interview, J.K. Rowling revealed that it was the murder of Bertha Jorkins that made Nagini a Horcrux.