The Bloody Baron

The Bloody Baron was a wizard who attended Hogwarts School of Witchcraft and Wizardry in the Founders' time and was sorted into Slytherin House.

After Hogwarts, he fell in love with Helena Ravenclaw, but she did not love him in return. He was sent by Rowena Ravenclaw to find Helena in Albania, but Helena refused to return with him and so he killed her in a rage. When he realised what he had done, the Baron stabbed himself with the same knife and died, committing suicide. (Also see the article Helena Ravenclaw (The grey lady)).

After he died he became The house ghost of Slytherin House.

Helena Ravenclaw: https://hogwartswiki.fandom.com/wiki/Helena_Ravenclaw_(The_Grey_Lady)