bluebeard

n. The hero of a mediæval French nursery legend, who, leaving home, enjoined his young wife not to open a certain room in his castle. She entered it, and found the murdered bodies of his former wives. -- Also used adjectively of a subject which it is forbidden to investigate. [ 1913 Webster ]

The Bluebeard chamber of his mind, into which no eye but his own must look. Carlyle. [ 1913 Webster ]