Sentence:  “The story of this adoption is, of course, the pivot round which all the circumstances of the mysterious <WSD>tragedy</WSD> revolved. Mrs. Yule had an only son, namely, William, to whom she was passionately attached&thinsp;; but, like many a fond mother, she had the desire of mapping out that son's future entirely according to her own ideas. [...]" 
Meaning of the word marked with <WSD> tags:  A disastrous event, especially one involving great loss of life or injury.