Underground-dwelling women of Sigma Draconis VI who stole Spock's brain to run the machine that sustained their civilization.
The Eymorgs are the female half of the people of Sigma Draconis VI. They live in an underground complex maintained by a great machine called the Controller, while the men, the Morgs, live a primitive life on the frozen surface. The Eymorgs themselves have lost most of their ancestors' knowledge and rely on a device called the Teacher to gain it temporarily.
Their leader Kara boarded the Enterprise and removed Spock's brain to serve as the new Controller. McCoy used the Teacher to restore it, and Kirk told the Eymorgs that they would have to learn to live with the Morgs.