Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
16. The Musgrave Ritual
Here's another revelation. After killing Redbeard, Eurus repeated a strange song that apparently held the key to finding him. Mycroft calls this her "ritual." She then set the Holmes' family house alight - a house called Musgrave Hall.
This is a loose reinterpretation of Doyle's The Adventure of the Musgrave Ritual. In that story, a college friend of Holmes', Reginald Musgrave, asks the detective to investigate an incident involving the family's traditional riddle.
Eurus's song and Doyle's riddle are different but they have several lines in common. Eurus sings of "16 by 6 and under we go", for example, while Doyle wrote "North by ten and by ten, east by five and by five, south by two and by two, west by one and by one, and so under.”