He defiled it! With Dark magic!

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 third 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.