Rhea of the Cöos

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Rhea Dubativo of the Cöos is a witch in the fictional Dark Tower series by Stephen King. She apparently has a number of eldritch 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 killed her, but King never elaborated beyond this brief mention.