In BBC’s Sherlock, the prison island holding Eurus Holmes is called Sherrinford.
Sherrinford is introduced as a highly secure island facility where Eurus, Sherlock and Mycroft Holmes’s sister, has been confined. The prison is the setting for “The Final Problem,” in which Eurus manipulates a series of psychological tests involving Sherlock and his friends.
The name has a long history in Holmes scholarship. Sherrinford was an early name considered for Sherlock Holmes by Arthur Conan Doyle before the final character name was chosen. The television series turns that unused name into a location.
Sherrinford is not the name of Eurus herself, and it is not the same as the fictional prison Azkaban from Harry Potter. The island’s isolation and security emphasize the extraordinary danger attributed to Eurus within the BBC story.