Rowena Ravenclaw's diadem

"Yeah, but the lost diadem is lost...that's sort of the point."

- Michael Corner to Luna Lovegood

Rowena Ravenclaw's Diadem (also known as the Lost Diadem of Ravenclaw) was the only known relic once belonging to Rowena Ravenclaw, the founder of Ravenclaw House at Hogwarts School of Witchcraft and Wizardry. Etched upon its surface was Ravenclaw's famous quotation: "Wit beyond measure is man's greatest treasure." It was said to enhance the wisdom of its wearer, which is Ravenclaw House's most treasured attribute. The diadem was later stolen by her daughter, Helena, a fact Rowena kept a secret from the other founders till her death bed.

Rowena Ravenclaw's Diadem, Helga Hufflepuff's Cup and Salazar Slytherin's Locket, made up three of Lord Voldemort's Horcruxes.

Rowena Ravenclaw
As Rowena Ravenclaw was a very intelligent woman, it is most likely she enchanted the diadem herself, putting a charm on the object to increase the wearer's intelligence. Rowena died shortly after her daughter, Helena, stole the diadem.

Helena Ravenclaw
Helena Ravenclaw, Rowena's daughter, greatly envied the attention her mother received. She stole her mother's diadem, in hope of using its power to make herself wiser, and fled Hogwarts. Her mother fell fatally ill and in spite of Helena's betrayal, wanted to see her daughter one last time. Rowena sent the Bloody Baron, a man who once loved Helena, to find her. When he caught up with her in a forest in Albania (the same forest Lord Voldemort would use as a hiding place during his exile), she hid the diadem in a hollow tree. Upon her refusal to return with him, the Baron stabbed her; horrified by what he had just done, the Baron then stabbed himself as well. The diadem was later found to be lost (hence the name "the lost diadem"), and was sought after by students for hundreds of years.

Tom Riddle
"He defiled it! With dark magic!"

- The ghost of Helena Ravenclaw to Harry Potter

Centuries later, Tom Marvolo Riddle managed to charm the story out of Helena's spirit, then known as the Grey Lady, ghost of the Ravenclaw house. Upon learning of the diadem's location, Riddle travelled to the Albanian forest shortly after he left school and acquired the diadem. He transformed it into his fifth Horcrux by killing an Albanian peasant, and later chose to return it to the very place it had originated from: Hogwarts.

Riddle was arrogant enough to believe that he alone had penetrated Hogwarts's most mysterious secrets, and thus he believed that only he had discovered the Room of Requirement, where he intended to hide the diadem. The Room's manifestation was none other than the "Room of Hidden Things" inside which Harry Potter would hide the Half-Blood Prince's copy of "Advanced Potion-Making" many decades later. Riddle chose to hide the diadem in this room the night he returned to the castle to request a position to teach Defence Against the Dark Arts. Unlike the other Horcruxes, Riddle did not put up heavy magical protections for the diadem, due to his lack of time to do so, along with his arrogance.

Harry Potter
The diadem remained undisturbed in the "Room of Hidden things" for many decades, until it was sighted by Harry Potter during his sixth year as a "discoloured old tiara", though he did not realize what it was. In 1998, accompanied by Ron Weasley and Hermione Granger, Harry returned to Hogwarts in search of an object belonging to Ravenclaw, having learned that through the mental connection he held with Voldemort that it was hidden at the school. He was told of her lost diadem by several Ravenclaw members of Dumbledore's Army and managed to force the story out of the Grey Lady before figuring out exactly where it was hidden. He was confronted by Draco Malfoy, Gregory Goyle, and Vincent Crabbe before he could reach the diadem, and a battle ensued within the Room of Requirement, resulting in Crabbe's creation of Fiendfyre (a cursed fire which is of dark magic) that then went out of his control and resulted in his death. Harry was able to acquire the diadem, but was saved the trouble of having to destroy it, since the Fiendfyre had accidentally incinerated it. The diadem crumbled to pieces in his hands upon their escape from the Room of Requirement.

Behind the scenes

 * It is highly likely that the diadem will be one of the collectables released by the Noble Collection with the release of Harry Potter and the Deathly Hallows: Part 2.
 * In the books, Voldemort is unaware when one of his Horcruxes is destroyed; in the movies, he "feels" when one is destroyed, which triggers physically painful visions of Voldemort's thoughts for Harry (as Harry is a proto-Horcrux/is host to a part of Voldemort's soul); this is how Harry is able to deduce that one of the Horcruxes is tied to Ravenclaw, since Voldemort's thoughts about the castle and Rowena Ravenclaw/[the safety of] the Ravenclaw artifact tranferred to him.
 * In the novel, the diadem is destroyed by Vincent Crabbe. In Harry Potter and the Deathly Hallows: Part 2, Harry stabs the diadem with a basilisk fang and Ron kicks it to the Fiendfyre (then created by Gregory Goyle) inside the Room of Requirement. Right before the door of the room closes, Voldemort's face appears in the fire, because a part of soul came free.
 * Since the diadem was a piece of jewelry, it is highly likely Voldemort put curses on the diadem after turning it into a Horcrux if anyone would try to wear it, similar to the curses and side effects of wearing either the ring or the locket.
 * The Grey Lady calls the Diadem an abomination due to it being tainted by the Dark Magic of a Horcrux. Indeed, when Harry finds it, rather than making the chittering violent noises of the others it makes a wailing noise as if the Light and dark forces inside of it are not sitting well together.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * The Wizarding World of Harry Potter