From the preface:

We were talking last evening-as the blue moon-mist poured in through the old-fashioned grated window, and mingled with our yellow lamplight at table-we were talking of a certain castle whose heir is initiated (as folk tell) on his twenty-first birthday to the knowledge of a secret so terrible as to overshadow his subsequent life. It struck us, discussing idly the various mysteries and terrors that may lie behind this fact or this fable, that no doom or horror conceivable and to be defined in words could ever adequately solve this riddle; that no reality of dreadfulness could seem caught but paltry, bearable, and easy to face in comparison with this vague we know not what.