So, the moment Harry cast Expelliarmus, it collided with Voldemort's own Killing Curse, the spells connected, Elder Wand finally realized and recognized Harry as its true master, and made the curse rebound. The Killing Curse rebounded on Voldemort, hence finally killing him.
Harry's spell simply rebounded a weak killing curse back onto Voldemort who was extremely weakened with all of his Horcrux's destroyed. The Elder wand couldn't conjure a full power curse for Voldemort.
Harry became so enamoured with the spell that he eventually used it to finish off Lord Voldemort. Here's how Harry's relationship with Expelliarmus became his signature – and why that's a good and bad thing.
There is one thing that makes Expelliarmus surprisingly powerful. It can make wands change allegiances. This means that even if the enemy wizard gets their wand back, they would have trouble fighting for a second time.
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.
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…'
When Snape killed Dumbledore. In probably the most shocking twist of the series, Snape cast the unforgivable Avada Kedavra curse on Hogwarts' beloved headmaster.
Alohomora, which is a spell that unlocks objects (and it's also Hermine Granger's signature spell).
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.
I've always suspected Molly's lethal spell was a particularly well-placed and powerful stunner that stops Bellatrix's heart, which is consistent with the description of her death: Molly's curse soared beneath Bellatrix's outstretched arm and hit her squarely in the chest, directly over her heart.
Who is immune to Avada Kedavra? Phoenixes were semi-protected from the Killing Curse, due to them being immortal. In 1996, Fawkes swallowed one intended for Albus Dumbledore, causing him to burst into flame and die instantly. However, he then was reborn from his ashes.
When Harry wrestles Draco's “everyday” wand out of his hand at the Malfoy's mansion, he conquers Draco, and therefore the Elder Wand — hidden in Dumbledore's tomb at the time — transfers its allegiance to Harry. Rowling said her American editor suggested the moment when Harry conquers Draco should be more dramatic.
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.
Grindelwald refused to tell him, so Voldemort murdered him with the Killing Curse. Grindelwald was responsible for a lot of death when he was raising his army of wizards.
Hermione's bluebell flames
Hermione first uses it against Snape when she and Ron think he is cursing Harry's broom in his first Quidditch match. Her bright blue flames set Snape's robes alight, before Hermione scoops them up and puts them into a jar.
Snape save Draco by using Vulnera Sanentur spell. 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 intentions of using it against his enemies and it soon became one of his specialties."
7 Draco Malfoy: The Protean Charm.
Hermione's bluebell flames
Hermione's magical talents meant she was good at every spell, right from the beginning – Wingardium Leviosa, anyone? But the bluebell flames she once used against Snape became one of her trademarks, and it's also one of our favourites.
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.
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.
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.
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.