Its like a crown or a tiara inscribed with the words "Wit, beyond measure, is man's greatest treasure." Rowena Ravenclaw used to own this when she was alive. It it one of the horcruxes. It is destroyed by Fiendfyre, or cursed fire, in the last book.
When Rowena Ravenclaw owned the diadem, her daughter Helena Ravenclaw (who is now the Ravenclaw ghost) stole it and ran away. Rowena sent the Bloody Barron to bring her back. When he found her he asked Helen to come back, she refused, and then out of rage the Bloody Barron killed her. After he realized what he had done, he killed himself (he is now the Slytherin Ghost).
wit beyond measure, is man's greatest treasure
Well, actually Rowena Ravenclaw's magical Diadem was stolen by her own daughter - The Grey Lady - the ghost of Ravenclaw House. After that The Grey Lady flees. Rowena asks The Grey Lady's lover to go & find her but instead he kills her. But before that The Grey lady manages to hide The Diadem in a stump of a tree by magic. The Grey Lady's lover was The Bloody Baron - the ghost of Slytherin House. He finds and kills her, I don't remember why. That's the reason he has blood on his robes. Many years later Tom Riddle discovers from The Grey Lady the location of the Diadem, finds it and turns it into a Horcrux.
Vincent Crabbe destroyed the diadem when he used fiendfyre (cursed fire) on it, while trying to kill Harry Potter (along with Ron Weasley and Hermione Granger).
The diadem (a crown) which belonged to Rowena Ravenclaw. It was stolen by her daughter, Helena Ravenclaw and wasn't found again until Lord Voldemort went looking for it in Albania. Voldemort then turned it into a Horcrus and left it in the Room of Requirement at Hogwarts where it remained until it was destroyed in a fire during the battle of Hogwarts.
Rowena Ravenclaw's lost diadem
The Ravenclaw house ghost it the Grey Lady. Her human name was Helena Ravenclaw who was the daughter of Rowena Ravenclaw who founded Ravenclaw house. Helena stole her mothers diadem and hid in Romania. Her mother sent the many who loved her, the Bloody Baron, to find her. She refused to go with him so he killed her then himself and that is why he wears his chains and is covered in blood. Tom Riddle convinced Helena to tell him where the diadem was then he turned it into a Horcrux which led Harry to the Room of Requirement where all hidden things were.
There is no Rowena Ravenclaw in the Harry Potter movies, only her daughter, Helena Ravenclaw.
There are 7 horcruxes. The diary of Tom Riddle, the cup of Helga Hufflepuff, the diadem of Rowena Ravenclaw, the locket of Salazar Slytherin, the ring which belonged to Voldemort's grandfather, the snake Nagini, and one was inside Harry.
Rowena Ravenclaw was not in the Harry Potter movies. Her daughter, Helena Ravenclaw was. She was played by Kelly Macdonald.
Yes, the Bloody Baron did commit suicide after he unwillingly murdered Rowena Ravenclaw's daughter, Helena, after Rowena herself sent for her while she was lying on her deathbed. Her daughter refused to come because she had stolen her mother's tiara or diadem, though her mother knew but never brought it up after it had been gone for years. answered by RoSeWeAsLeY
He remember's he was it in "the room Of Hidden things/ The come and go room / the room of requiment.
**Little Spoiler** In the 7th book Helena Ravenclaw (Rowena Ravenclaw's daughter) told Harry that her mother became fataly ill and died.