Sentence:  “The story of this adoption is, of course, the <WSD>pivot</WSD> round which all the circumstances of the mysterious tragedy 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:  Something or someone having a paramount significance in a certain situation.