Rowena Ravenclaw's diadem

Rowena Ravenclaw's diadem also known as the lost diadem. It is an heirloom which belonged to Rowena Ravenclaw, the celebrated Hogwarts Founder of Ravenclaw House. In appearance, 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 House's most treasured attribute.

Helena Ravenclaw
Helena Ravenclaw, Rowena's daughter, greatly envied the attention her mother received to a point where she stole her mother's diadem, in the hopes of using its power to make herself wiser, and fled Hogwarts. She was pursued by her then-suitor, the Bloody Baron. Shortly before he caught up with her in a forest in Albania (the same forest Lord Voldemort would use as a hiding place during his exile far in the future), she hid the diadem in a hollow tree. When she refused his advances, the Baron stabbed and fatally wounded Helena by accident. Horrified by what he had just done, he stabbed himself to death as well. The Diadem has been lost, and sought after, for hundreds of years since.

Tom Riddle
Centuries later, a single Hogwarts student successfully pretended to sympathize 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 travelled 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 (killing an Albanian peasant in the process), 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 the Room of Requirement. 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. 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.

Harry Potter
There the diadem remained undisturbed for many decades until it was sighted (and ironically picked up and moved) by Harry Potter during his sixth year, though he did not realize what it was at the time much like Salazar Slytherin's locket in Number 12 Grimmauld Place.

In 1998, accompanied by Ron Weasley and Hermione Granger, Harry returned to Hogwarts in search of an object belonging to Ravenclaw, (correctly) believing it to be one of Voldemort's Horcruxes. He was quickly told of her lost diadem and managed to force the story out of the Grey Lady before figuring out exactly where it was hidden. Just as he was about to claim the Horcrux diadem, he was confronted by Draco Malfoy, Gregory Goyle, and Vincent Crabbe. A battle ensued throughout the Room of Hidden Things, resulting in Crabbe's accidental creation of Fiendfyre, or cursed fire, which 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 Hidden Things.