The Adventure of the Veiled Lodger

Mrs. Merrilow of South Brixton's lodger, Mrs. Ronder, is usually very quiet. The woman never shows her face and with reason, for it is severely mutilated - Mrs. Merrilow only saw it once. But lately she has started shouting in her sleep and her health has been going downhill. She refuses help from anyone, except Holmes, and charges Mrs. Merrilow, when she brings her case to him, to mention "Abbas Parvas".
The detective knows just what that means. "The Adventure of the Veiled Lodger" is part of "The Case-Book of Sherlock Holmes".