At some point in time, Lord Voldemort gave Bellatrix Lestrange Hufflepuff's Cup, one of his Horcruxes, to keep safe in her vault, though he didn't tell her its true nature.
The mind-controlled goblin places his palm to the door of the vault, and it opens. Harry knows from his conversations with Dumbledore that if a Horcrux is in the vault, it must either be the Hufflepuff cup or else an unknown object, so they have no choice but to look for the cup.
Many years later he left it in the care of his trusted Death Eater, Bellatrix Lestrange. Although she did not know that the cup was a Horcrux, Lestrange treated the object with extreme priority, storing it in her family vault at Gringotts Wizarding Bank, considered to be impossible to plunder.
Griphook and an Imperiused Bogrod helped Harry, Ron and Hermione break into Bellatrix's vault and recover the Horcrux. The cup was destroyed by Hermione Granger, using a fang from the Basilisk's corpse.
The group was caught and punished as a result. Following that incident, Severus Snape passed the counterfeit sword to Bellatrix Lestrange. Bellatrix then stored it in her Gringotts vault alongside Helga Hufflepuff's cup, one of Voldemort's Horcruxes.
Inside the Lestrange's Vault
When Harry, Ron and Hermione venture inside Bellatrix Lestrange's vault in search of a Horcrux, they receive an unpleasant surprise: The contents of the vault are under the Geminio curse, so everything they touch multiplies exponentially.
Bellatrix Lestrange
Ollivander described this wand as "unyielding" a very telling feature of the owner as well. Legend has it that there was a European superstition warning not to grow anything near a walnut tree because it contained evil or poison-also in the walnut-wood wand of Bellatrix.
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.
The Flagrante Curse and Gemino curse were placed on the treasure inside so that everything touched would burn and then multiply until the would-be thief was crushed to death by the amount of expanding gold if he or she kept trying to touch objects.
The sword was supposed to be in her vault in Gringotts. Voldemort assigned that job to her; to keep this sword, and a few other valuable items (e.g. unknown to her, another Horcrux). So when she saw the sword in Hermione's hand, she got really scared and feared the worst. She thought her vault has been breached.
His reaction to Bella's death was akin to his reaction at the loss of the Cup, which was when he realised that his own immortality was jeopardised amidst war and his own death could have been near.
He did not love anyone, and did not care to be loved. He felt nothing for his family, and only wanted his friends in school to rally around him because he wanted to hold power over them. Bellatrix Lestrange was obviously in love with him and worshiped him, but he only cared for her servitude, not her.
No, we don't know what spell did Molly use to take down Bellatrix, but it certainly wasn't Avada Kedavra. Whatever spell she used, it didn't disintegrate Bellatrix either.
Draco's refusal to reveal Harry's identity to Bellatrix was not because he liked him. It was because he believed that Harry was the only chance they had at defeating Voldemort. At first, Draco admired and revered Voldemort until he threatened his family's safety.
Because Bellatrix's wand was reported as lost at the Malfoy Manor where the trio ended up taking it away to Shell Cottage, so showing it would have immediately revealed that Hermione was only Bellatrix in disguise, as the real Bellatrix didn't have her wand.
Voldemort, weakened by his long stint as "less than the meanest ghost' 69 insists on making one more Horcrux. And unlike with Harry, Voldemort did say the spell when he killed Bertha Jorkins to make Nagini a Horcrux.
But when Ron and Hermione are about to die by Nagini, Ron casts the killing curse as a last resort and it simply rebounds off Nagini as it was nothing.
History Of Avada Kedavra
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel.
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.
Narcissa Malfoy's Very Dangerous Lie
But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
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.
Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
Phoenix. This is the rarest core type. Phoenix feathers are capable of the greatest range of magic, though they may take longer than either unicorn or dragon cores to reveal this. They show the most initiative, sometimes acting of their own accord, a quality that many witches and wizards dislike.
Elder. The rarest wand wood of all, and reputed to be deeply unlucky, the elder wand is trickier to master than any other. It contains powerful magic, but scorns to remain with any owner who is not the superior of his or her company; it takes a remarkable wizard to keep the elder wand for any length of time.