Rhea Dubativo of the Cöos is a fictional character who appears in several novels by Stephen King, most notably his Dark Tower series, as well as the comic book spinoffs of that series. Rhea is a very stereotypical fairy tale witch in appearance, personality and abilities; she is old, ugly, rides a broomstick and has a cat familiar. In The Dark Tower IV: Wizard and Glass, she is said to represent the Wicked Witch of the West from The Wizard of Oz.
She apparently has a number of eldritch powers, including but not limited to the ability to own familiars, has the power of prophecy to a minor extent, can see visions through crystal balls, make magical potions, and be immune to powerful venom. She also has some knowledge of medicine, as she was able to determine that Susan Delgado was a virgin. In The Eyes of the Dragon, she is described as riding a broomstick, like stereotypical witches.
Rhea is first mentioned in King's 1987 fantasy novel, The Eyes of the Dragon, although she has a very minor part in the story.Those who live in the fictional kingdom of Delain, evoke her as a boogeyman whose appearance riding on a broomstick is a harbinger for disaster. She is spotted shortly before a devastating snowstorm strikes Delain during the novel's climax. Very possibly she is acting together with Flagg, whom we know to have contacted her as Marten Broadcloak.
Rhea has a larger part in the The Dark Tower IV: Wizard and Glass, the fourth novel in the series. As the book progresses, her repeated usage of the Pink Bend (a.k.a. Maerlyn's Grapefruit) of the Wizard's Rainbow drains her, causing her to become progressively older and more evil. Her image coaxes Roland Deschain to kill his own mother through illusion, but it's likely that the Crimson King is using her image, not Rhea herself. Rhea was always a contrary child, and did evil things to her family and the children around her. A piece of Maerlyn's glass from his mirror (which showed people's true selves) lodged itself in Rhea's eye as a youth, enabling her with the sight and enhancing her powers (much like Kaj from The Snow Queen).
Roland implies at the end of Wizard and Glass that he did indeed meet up again with her on his travels and kill her, but King has never elaborated beyond this brief mention.
More of her background is mentioned in the Dark Tower comics.
Upon her deathbed, she managed to avoid dying by agreeing to go to Mejis, (Susan Delgado's hometown) and preventing Roland's child from being born. It was revealed to Rhea that under no circumstances must a child be born of Roland's blood. This prophecy reveals that Roland would bring down the Crimson King. Rhea prevents this birth by instigating the burning of Susan, who was in fact, with Roland's child.