Rowena Ravenclaw's diadem

Rowena Ravenclaw's Diadem is an heirloom which belonged to Helena Ravenclaw, the jealous daughter of the the celebrated Hogwarts Founder of Ravenclaw House. The Diadem was not too unlike the shimmering tiara of Muriel Weasley, and etched upon its surface was Ravenclaw's famous quotation: "Wit beyond measure is a man's greatest treasure." It was said to enhance the wisdom of its wearer, which is Ravenclaw's most treasured attribute.

Helena Ravenclaw, who was being courted by the Bloody Baron, stole her mother's diadem and fled. She was pursued by her then-suitor, the Bloody Baron. Shortly before he caught up with her in a forest in Albania, she hid the Diadem in a hollow tree. When she refused his advances, the Bloody Baron stabbed and fatally killed Helena by accident. Horrified by what he had just done, he stabbed himself as well, resulting in his untimely death.

Centuries later, a single Hogwarts student sympathized with Helena's lust for powerful objects and her story in general, and this student was none other than the charming Tom Marvolo Riddle. He charmed the story out of Helena's spirit, now known as the Grey Lady. Upon learning of the Diadem's location, Riddle traveled to the Albanian forest Helena had told him about shortly after he left school and acquired the Diadem. He then transformed it into one of his many Horcruxes, and 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 a certain room he had discovered had only been discovered by himself, that room being none other than the Room of Requirement. The Room's form was none other than the "Room of Hidden Things" that Harry Potter would hide the Half-Blood Prince's copy of "Advanced Potion-Making" many decades later. He 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. 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 at the time, much like Salazaar Slytherin's Locket.

Harry finally discovered upon further research and guesswork what the Ravenclaw Horcrux was and exactly where it was hidden. Just as he was about to claim the Diadem Horcux, he was confronted by Draco Malfoy, Gregory Goyle, and Vincent Crabbe. A battle ensued throughout the Room of Hidden Things, resulting in Crabbe's manifestation of Fiendfyre, or cursed fire, which resulted in his death. Harry was able to acquire the Diadem Horcrux, but was saved the trouble of having to destroy it as the Fiendfyre had gotten to it, and it crumbled to pieces in his hand upon their escape from the Room of Hidden Things, moving him one step closer to making Lord Voldemort susceptible to death once more.