Final answer:
The disgraced butler in 'The Musgrave Ritual' is B. Brunton. He is crucial to the narrative as his actions initiate the mystery that Sherlock Holmes ultimately unravels.
Step-by-step explanation:
The disgraced butler at the center of "The Musgrave Ritual" is B. Brunon. The story is one of the Sherlock Holmes detective stories written by Sir Arthur Conan Doyle. In this story, Brunton, the butler of the Musgrave family, is found prying into the family's private documents and is subsequently dismissed. However, he is central to the mystery as his discovery of an old ritual and his subsequent actions spark off the events that lead Holmes to solve the case.