There were noted to had been fourteen such devices each of which were identical to one another in size and shape. Their appearance was larger than two Humanoid fists together but were flat and copper colored. On the top were four keys; two of which were amber while one was brown and the last was a deeper shade of brown. The resonators were cool to the touch and felt light with a number of indentations on the other side. These keys had the capacity to shut down their Iconian gateway network. This aspect of their technology was built to last and was still functional even after a thousand years had passed.
These devices worked by holding them over the command panel of the gateways and lowering them in a way that fit over the control keys. The resonators fit in snugly atop a cluster of amber and blue keys to the right side of the console. Once in place, the resonator keys glowed with other keys lighting up and a thrum of power emanating from the equipment. After half a minute, a small ball of light formed above the resonator with one of the amber keys blinking. The light grew in size and altered in shape where it formed a sphere that engulfed the top of the control panel. Inside the sphere were smaller swirls which formed into a representation of the universe with galaxies forming. Items of note were represented in purple lights.
In order to initiate a complete shut down of the gateway system, thirteen resonator keys were required to be placed in their respective gateways. Once complete, the Master Resonator worked only on the biosignature of the one who made first contact with it which prevented further use by other parties.
At some point in their past, the scattered Iconian Empire deposited the Master Resonator on one of the worlds they had visited where it was held by a native primitive species. In 2376, the Sentries agreed to help Starfleet Captain Jean-Luc Picard to shut down the gateway network which had been activated by the Petraw and was causing havoc across the galaxy. However, their records informed them that they could only do this with the resonator and dispatched Picard to the planet in order to recover it.
After uncovering the device, he learned that thirteen of the keys were needed at various gateways to deactivate the system. Thus, he traveled back to known space and worked in conjunction with the crews of various starships who placed the resonators at their respective stations which ended the Gateway crisis. (TNG - Gateways short story: "The Other Side")