Rowena Ravenclaw's diadem

Rowena Ravenclaw's Diadem (also known as the lost diadem) is the only known relic which once belonged to Rowena Ravenclaw, the founder of Ravenclaw House in the celebrated 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. It was later stolen by Rowena's daughter, Helena, a fact which she then kept a secret from the other founders till her death bed.

Along with Helga Hufflepuff and Salazar Slytherin's relics, they made up three of Lord Voldemort's Horcruxes.

Rowena Ravenclaw
As Rowena Ravenclaw was a very intelligent woman, it is most likely that she created the diadem. She likely wanted to make herself the wisest woman (and person) in the world. She probably put a charm on the diadem to make the wearer very clever. Shortly after her daughter, Helena, stole the diadem, she died.

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 her betrayal, she wanted to see her daughter one last time. So she sent a man who had once loved Helena, the Bloody Baron. He shortly 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. When she refused to return with him, the Baron stabbed her. Horrified by what he had just done, he stabbed himself as well. The diadem was later found to be lost (hence the name "the lost diadem"), and sought after by students, for hundreds of years since.

Connections with Tom Riddle
Centuries later, Tom Marvolo Riddle managed to charm the story out of Helena's spirit, now 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 then transformed it into one of his Horcruxes by killing an Albanian peasant. Later, he 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.

Conections with Harry Potter
There the diadem remained undisturbed for many decades until it was sighted by Harry Potter during his sixth year, 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 seen it from his mind connection with Voldemort. He was told of her lost diadem by Ravenclaw students (Luna Lovegood) 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 could reach the diadem. A battle ensued throughout 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, and it crumbled to pieces in his hands upon their escape from the Room of Requirement.

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 II