A small, advanced ion-propelled starship used by the Eymorg leader Kara to intercept the Enterprise and steal Spock's brain, leaving an ion trail that Kirk follows to Sigma Draconis.
The Eymorg ship appears at the start of the third-season Original Series episode Spock's Brain. It approaches the Enterprise in deep space, and Scott is openly impressed by its design and by an ion drive far beyond anything Starfleet can build.
Its single occupant, Kara, beams directly onto the bridge, stuns the whole crew with a wrist control and surgically removes Spock's brain before departing. The ship is not seen again, but its ion trail gives Kirk the only lead he has.
The Enterprise follows the trail to the Sigma Draconis system, where the sixth planet proves to be the home of the Eymorg and of the Controller that the stolen brain has been installed to run. The vessel's sophistication contrasts sharply with the Eymorg themselves, who no longer understand the technology their ancestors left them.