Ravenclaw's Diadem and the Room of Requirement: Horcrux 5

There's a room at Hogwarts that only appears when you truly need it. Voldemort found it as a student and was certain he was the only one who knew its secret. He hid a piece of his soul there.

Half of Hogwarts knew about it. Students had been hiding forbidden things there for a thousand years. It was the dumbest mistake a genius ever made.

A Diadem Nobody Had Seen

Rowena Ravenclaw, founder of the house of the clever, had a diadem that, according to legend, gave wisdom to whoever wore it. It was lost centuries ago. To nearly everyone it was a myth. At the Lovegoods' house, Harry sees a bust of Rowena wearing Xenophilius's reconstruction: the diadem with wings, radishes and an ear trumpet (Deathly Hallows, ch. 20). That's how little was known about the real one.

Helena, the Daughter Who Stole It

The real story is told by a ghost. The Grey Lady, ghost of Ravenclaw tower, confesses to Harry during the battle who she was in life (Deathly Hallows, ch. 31). She was Helena, Rowena's daughter. She stole the diadem from her mother because she wanted to be cleverer than her, and fled to a forest in Albania.

Rowena, ill, sent a man who was in love with Helena to find her. She rejected him. Furious, he killed her, and then killed himself in remorse. That man is the Bloody Baron, Slytherin's ghost, who still drags chains as penance. Helena hid the diadem in the hollow of a tree before she died.

Tom, the Only One Who Listened

A thousand years later, a Slytherin student went to talk to Ravenclaw's shy ghost. Polite, understanding, interested. Helena hadn't told her story to anyone in centuries. She told Tom Riddle.

Tom went to Albania, found the diadem and turned it into a Horcrux. When he came back to Hogwarts to ask for a teaching job, which Dumbledore refused, he used the visit to hide it in the castle. Harry deduces it in the battle (ch. 31), putting the pieces together.

The Room of Requirement

The room takes many shapes. For Dumbledore's Army it was a practice hall. For Tom it was the place where things get hidden: a cathedral of objects abandoned over centuries, with forbidden books, broken furniture, old potions and thousands of student secrets.

Tom believed he was the only one to find it. It never occurred to him that a thousand years of students had hidden their things there before him.

The Horcrux Harry Touched

The biggest irony is in Half-Blood Prince (ch. 24). After the Sectumsempra incident with Draco, Harry runs to hide the Half-Blood Prince's book in that same room. To remember the spot, he puts a wig and an old, faded diadem he found lying around on an old bust.

It was Ravenclaw's diadem. Harry held a Horcrux in his hands a year before he knew what he was looking for.

Fiendfyre

During the battle, the trio goes back to the room. Draco, Crabbe and Goyle are waiting for them. Crabbe, who no longer even obeys Draco, casts Fiendfyre: living flames shaped like serpents, dragons and chimeras, which can't be put out and devour everything (Deathly Hallows, ch. 31).

The fire destroys the diadem. Crabbe can't control it and dies in it. Harry and Ron fly back in on brooms through the flames and pull out Draco and Goyle. Hermione helps them. They saved the life of the boy who made theirs miserable for seven years.

Book vs. Movie

The one who dies is Goyle, not Crabbe. The actor who played Crabbe couldn't film the last movie, so on screen Goyle casts the fire and dies.
How it's destroyed. In the movie Harry stabs the diadem with a basilisk fang and Ron kicks it into the fire. In the book the fire destroys it on its own.
The Grey Lady gets a longer scene in the movie than most ghosts, but her history with the Bloody Baron is barely told.

What Almost Nobody Notices

Voldemort got the diadem by listening to a lonely ghost nobody listened to. It's the only thing we see him do right with anyone: pay attention. And he did it only to rob her.

There's a symmetry, too: a Slytherin student unknowingly destroys the Horcrux that the heir of Slytherin had hidden. Crabbe had no idea what he was burning.