Theft of Rowena Ravenclaw's Diadem

"I stole the diadem. I sought to make myself cleverer, more important than my mother. I ran away with it. My mother, they say, never admitted that the diadem was gone, but pretended that she had it still. She concealed her loss, my dreadful betrayal, even from the other founders of Hogwarts."

- Helena Ravenclaw

The thievery of Rowena Ravenclaw's diadem took place in the Founder's Age (possibly early 1000s). Helena Ravenclaw, the daughter of the Ravenclaw House's Founder Rowena Ravenclaw, was jealous of her mother's wisdom and fame. She wanted to be cleverer and more famous than her mother, so she stole her mother's Diadem and escaped. Rowena, who was badly ill, ordered the Baron, a bad-tempered man who loved Helena, to find her daughter. However, when the Baron found Helena in a forest at Albania, she refused to go back with him. The Baron killed her in rage. After seeing what he had done, remorseful of his action, he stabbed himself to death.

The Diadem's origin
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 quote: "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 has a blue sapphire shaped like an oval. 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.

Thievery of Envy
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.

Aftermath
"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' 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.