Shortly before the Battle of Hogwarts, Harry not only successfully used the Cruciatus Curse against Amycus Carrow, he also cast the Imperius Curse against two individuals during the Gringotts heist which led to Harry, Hermione, and Ron getting in the Lestrange vault.
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.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius.
Looking to embrace the Dark Arts in Hogwarts Legacy? Well, you'll be glad to know that you have the option to learn three of the most powerful dark spells in the wizarding world – also known as the Unforgivable Curses: Crucio, Imperio, and Avada Kedavra.
The three curses that are classified as Unforgivable are the Imperius Curse, the Cruciatus Curse, and the Killing Curse.
They were first classified as "Unforgivable" in 1717. They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
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.
How Expelliarmus Beat Voldemort's Avada Kedavra - Harry Potter Explained - YouTube.
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.
Avada Kedavra cannot be blocked as you correctly pointed out, except by physical objects as seen when Dumbledore duels Voldemort in the Order of The Phoenix, or due to the effects of Priori Incantatem as a result of wands with twin cores duelling as explained in Goblet of Fire.
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.
One reason that Harry lived after being hit by the Harry Potter Killing Curse in The Deathly Hallows is his mastery of the Elder Wand. There was a belief that obtaining the Deathly Hallows would grant the finder some sort of immortality, which is how Harry survived Avada Kedavra in the forest.
10 Harry Potter: Expelliarmus / Expecto Patronum
Throughout the entirety of the franchise, Harry is thrown into the most heat, and his skills as a wizard are constantly tested. That being said, his go-to spell has to be Expelliarmus.
Description and effects: The Killing Curse is a lethal curse that causes instantaneous death upon impact. Compatibility with Bellatrix's death: Bellatrix's sudden death aligns with the effects of the Avada Kedavra curse.
Avada Kedavra, also known as the Killing Curse, kills a person instantaneously and without injury. There is few counter for it, and only one person, Harry Potter, has ever survived it with Sacrificial Protection that has been endowed upon Harry Potter by His Mother Love for Him.
The author explicitly states that both women were fighting to kill, despite Molly refraining from casting the Unforgivable curse Avada Kedavra. The result of the duel actually comes about as a result of nothing more than spectacular accuracy on Weasley's part.
Draco Malfoy (Cruciatus and Imperius Curses)
Although the use of these curses is rightly unforgivable, it's important to note that Draco never used them for his own benefit. It was always the pressure from Lord Voldemort that forced Draco into his crimes.
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.
Defeating The Dark Lord with his signature spell
Disarming is about defence, not murder. Voldemort was a killer, Harry was not. It was an important distinction for Harry, who had learned so much about Voldemort and seen first-hand the ways in which they were similar.
The most likely reason why Lord Voldemort's nose disappeared and he devolved into such an evil-looking creature is that as he dabbled deeper into the Dark Arts, his appearance gradually became as distorted as what remained of his twisted soul.
Avada Kedavra
It's used to kill people, it's an Unforgivable Curse, and it was Voldemort's favourite, so it's going last. There's probably a big philosophical debate we could get into about violent curses like this – would this one have been okay to use on an evil dark wizard, for instance?
Those that are gifted with the power of flight are perhaps more rare than any other wizard that has any other power. Able to fly purely because of their own powers and not reliant on any type of enchanted item or spell at all, the number of these special wizards known to exist can be counted on one hand.
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.