Massacre at Malfoy Manor

"The boy has discovered our secret, Nagini. It makes us vulnerable. We must deploy all our forces now to find him. And you, my friend, must stay close."

- Lord Voldemort

The massacre at Gringotts Wizarding Bank was a short but violent skirmish at Gringotts on 1 May, 1998.

Background
In 1998, Harry Potter, Ron Weasley, and Hermione Granger broke into Gringotts in order to steal one of Lord Voldemort’s Horcruxes from the vault belonging to the Lestrange family. The trio were assisted by Griphook, a former employee of Gringotts, who agreed to help them in exchange for Godric Gryffindor's Sword, but double-crossed them.

The trio broke into the bank with Hermione disguised as Bellatrix Lestrange, Ron given a fake identity as a disguise, while Harry and Griphook under the Invisibility Cloak. They found their way into the vault and acquired Helga Hufflepuff's Cup, and in their attempt to escape ended up flying a half-blind dragon out of the bank in what would become a famous escape.

Massacre
On the same day as the break-in, Voldemort was informed by several goblins from Gringotts that the Trio had robbed the Lestrange vault. This caused Voldemort to panic, and when he was informed that a small golden cup had been stolen, he flew into a rage. Realising at that moment that Harry was hunting his Horcruxes, Voldemort unleashed the Elder Wand while Bellatrix Lestrange, the Malfoys, and Nagini watched. He smote each goblin on the floor in their turn until he and his followers were left alone amongst the dead.

Troubled, Voldemort listed all the locations of his Horcruxes in his mind, unaware that Harry was connected to his mind at the time. He unwittingly revealed that his last Horcrux was at Hogwarts Castle, just as Harry previously suspected. This caused Harry, Ron, and Hermione to venture to Hogwarts, setting the stage for the climactic Battle of Hogwarts.