Mrs. Dark was an Eidolon demon who, along with her sister, Mrs. Black, worked for the Magister, for whom they helped supervise several gambling dens for the Pandemonium Club. Collectively, she and Mrs. Black were referred to as the Dark Sisters, a title they reveled in.
Under orders from the Magister, and with support from Nate Gray, the Dark Sisters kidnapped Tessa Gray, who had just arrived at the docks of Southampton, and brought her to the their house in London. Mrs. Dark, being an Eidolon demon herself, was the perfect mentor for Tessa, a child of the Eidolon Greater Demon who had the unique ability to "become" who she "Changes" into and access their memories and thoughts. Together, she and her sister succeeded in teaching Tessa to perfect her power to Change, using Nate's safety as her motivator.
One night however, weeks after they abducted her, the local Shadowhunters tracked the ouroboros-marked dagger used to kill Emma Bayliss and found that the symbol was used by the Pandemonium Club, which eventually led them to the Dark Sisters. On the night Tessa was set to meet and marry the Magister, the Shadowhunters came; Will Herondale snuck into the house and released Tessa. Unwilling to lose Tessa, the sisters fought the raiding Shadowhunters but were ultimately overpowered, and they were all able to escape with Tessa.
Mrs. Black perished in the battle. Though she survived, she lost her job and favor with the Magister for losing Tessa and went into hiding at a mansion in Highgate, where she desperately looked for ways to resurrect her sister.
As a part of the Magister's plot to keep the Nephilim occupied and distract them from the truh, he tricked Will and Jem—who did not yet know the Magister's true identity—into thinking that the Sisters were working on the Magister's clockwork army. Mrs. Dark was preparing for a necromancy ritual for her sister when the pair arrived, and she mocked them for being tricked by Mortmain. She told them about the Magister's plans, but only because she intended to burn and kill them for him. During the confrontation, she remained in the middle of her protection pentagram while in her true form; however, she was killed when Will cut the support rope for a chandelier hanging above her, which fell and crushed her.
Mrs. Dark was tall and thin, her hair almost colorless. She, like her sister, wore bright clothes and gloves, used to hide their clawed hands—their demon marks. According to Will, she was not much to look at, and even her true demon form was an improvement.
Mrs. Dark's true form resembled a large, repulsive stone statue because of her hard, gray, stone-like skin. She had horns, three rows of mouths with greenish fangs, twisted limbs, clawed hands, and yellow eyes that burned with a sulfuric glow.
- Jem took in the cat, who eventually became known as Church, she intended to use for the ritual. Because of a part of the ritual she'd already performed, Church became immortal.