Dark Sisters is the term used by certain individuals to refer to Mrs. Black and Mrs. Dark. The Dark Sisters once lived in the Dark House with the clockwork servants they made with their employer, the Magister.
In 1878, the sisters abducted Tessa Gray under the orders of Mortmain. They taught her how to control her power to Change by means of torturous techniques, in addition to telling her that her brother, Nate, was in their captivity and would suffer if she did not comply.
When Will Herondale investigated the death of Emma Bayliss, he learned of the sisters and was lead to their home. The Shadowhunters of the London Institute then went to investigate further. The sisters chased after them and Tessa, who was making her escape with Will and the others. During the fight that ensued, Mrs. Black was beheaded and killed by Will, but Mrs. Dark escaped.
Mrs. Dark then hid at a mansion in Highgate. Mortmain, to distract the guardians of the Institute, tipped off Will and Jem about her whereabouts. They went to the mansion, where Mrs. Dark was about to attempt to use necromancy to bring her sister back to life. Shortly before she was killed, she revealed that Mortmain was the real Magister, not de Quincey.
Mrs. Black was resurrected by Mortmain shortly after. Her body no longer accounted for, she was given a temporary body, though Mortmain promised her an automaton's body if she continued to do as he asked. Obliging his orders, Mrs. Black orchestrated an automaton attack on the Institute and captured Tessa. However, after delivering Tessa to him, Mortmain killed her again to prove his sincerity to Tessa.
- There were records of two women from the 1600s who were believed to be the Dark Sisters, who were forced into seclusion because of the witch hunts that were widespread during the period.