Fortitude - The Grey Lady (HP Yarn Club November 2017)
Helena Ravenclaw, also known as the Grey Lady, was Rowena Ravenclaw's daughter, and a witch. She was sorted into Ravenclaw House upon entering Hogwarts in the 10th century.
Helena stole her mother's diadem and ran away to Albania. Rowena became fatally ill and, hoping to see her daughter one last time, sent the Bloody Baron, a man who harboured an unrequited love for Helena, to find her. The Baron murdered Helena in a rage when she refused to return with him, then committed suicide out of regret for what he had done. She and the Baron eventually returned as ghosts to Hogwarts Castle, where Helena became and continues to be the Ghost of Ravenclaw House.
"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, confessing to Harry Potter