She was the Queen of Falastur, the twelfth king of Gondor (TA 830-913). She was solitary and nefarious, and as she was scornful of beautiful things and colorful adornment, she dressed in only black and silver, and lived in bare chambers.

Queen Berúthiel had nine black cats and one white, her slaves, with whom she conversed. She influenced them to spy on the people of Gondor, enabling her to discover all of the dark secrets 'that men wish most to keep hidden'. No one dared touch the cats and all were afraid of and cursed them.

Eventually Falastur put Berúthiel on a ship alone with her cats and set them adrift on the sea before a north wind. The ship was last seen flying past Umbar under a sickle moon, with a cat at the masthead and another as a figure-head on the prow.

Though Berúthiel's name was erased from the Book of the Kings, the memory of her was never completely eliminated from the speech of men. While in Moria, Aragorn said of Gandalf that "He is surer of finding the way home in a blind night than the cats of Queen Berúthiel."

*Note*: the story of Queen Berúthiel is told only in an unpublished story which Christopher Tolkien included in Unfinished Tales so it may not be considered canonical by many, but since Aragorn does mention her in The Lord of the Rings, it would seem that the story does fit with Tolkien's intentions. You can be your own judge.