An Instrument of Obedience was a device used on board the Fabrini world-ship Yonada. An implant placed just below the skin near the temple, the device was used by the Oracle of the People to control the thoughts and behavior of the Fabrini on the ship.
The devices were invented by the Creator Tomaneru Vari. Initally, the devices were only implanted in people who had proven that they could not or would not obey the law on Yonada. After a series of violent upheavals several thousand years after the launch of Yonada, a fundamentalist regime came into power. The Oracle was reprogrammed with the fundamentalist ideology, and the devices were implanted in everyone on Yonada. (TOS novel: Ex Machina)
Using the Instruments, the Oracle was able to inflict pain on or even kill those who behaved in a way that it did not approve of. When Dovraku's father complained of his situation, he would often feel the pain the Oracle inflicted upon him. The Oracle killed Natira's father when he tried to reveal the truth of Yonada to his young daughter. Later, when an old man attempted to tell Captain James T. Kirk that he had discovered the truth behind Yonada, the Oracle used the device to kill him. The Oracle also attempted to kill both Doctor Leonard McCoy - who had been implanted with one of these devices when he married Natira - as well as Natria herself. Commander Spock removed the Instruments from both McCoy and Natira. (TOS episode: "For the World is Hollow and I Have Touched the Sky", TOS novel: Ex Machina)
The devices were removed from the Fabrini when the Oracle was shut down. The newfound freedom, and the reaching their final destination, led to further upheaval amongst the Fabrini people, some of whom responded by seeking to return to being ruled by the Oracle. (TOS novel: Ex Machina)