A small device implanted beneath the skin of every inhabitant of Yonada, through which the Oracle monitors its people and inflicts pain or death on those who question its teaching.
The Instrument of Obedience is a subcutaneous implant placed in the temple of each of the people of Yonada, the asteroid ship built by the Fabrini. It appears in the original series episode For the World Is Hollow and I Have Touched the Sky.
Through the implant the Oracle, the computer that rules Yonada, listens to its people and punishes forbidden thoughts and words. An old man who tells the landing party that the world is hollow is killed by his instrument moments later.
McCoy accepts an implant when he chooses to remain on Yonada and marry the high priestess Natira. When he tries to tell Kirk about the Fabrini book of knowledge the device nearly kills him, and Spock removes it. Natira's own instrument punishes her when she begins to doubt.