Rhea of the Cöos

Rhea Dubativo of the Cöos is a witch in the fictional Dark Tower series by Stephen King. She apparently has a number of powers, including but not limited to the ability to own familiars, see though crystal balls, make potions, and be immune to powerful venom. She also seems to have some knowledge of medicine, given that she was able to determine that Susan Delgado was a virgin.

Rhea was introduced in "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. She coaxes Roland Deschain to kill his own mother through her powers of illusion, in retribution for killing one of her familiars, her pet snake Ermot.

Roland implies at the end of "Wizard" that he did indeed meet up again with her on his travels and kill her, but King never elaborated beyond this brief mention. It is possible that this may be expanded on in the comic series.

More of her background is mentioned in the Dark Tower comics. 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. 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 lets us know that from the beginning, it was deemed that Roland would bring down the Crimson King. Rhea prevents this birth by instigating the burning of Susan, who was in fact, kindled with Roland's child. Her promise fulfilled, she took it one step further, arrived in Roland's home and tricked into killing his own mother. This matricide is Rhea's final revenge, for Roland killed her beloved snake, Ermot.