Eurus Holmes is the sister of Mycroft and Sherlock Holmes. She was completely unknown until her character was revealed in 'The Lying Detective'. Her intellectual capabilities exceed that of both Mycroft and Sherlock. She has the ability to manipulate minds and convince people to do her bidding. She is a master of disguise. She was holed up in the maximum-security psychiatric prison, Sherrinford, situated on an island on the orders of Mycroft, unbeknownst to her parents. As a child, she trapped Sherlock's best friend in a well and gave Sherlock clues to his whereabouts in a song. Later, she set fire to the family house after which she is sent to Sherrinford