A Starfleet ship lost at the planet Beta III about a century before the Enterprise arrived to learn what had become of it.
The USS Archon was a Starfleet vessel that visited the planet Beta III about a century before the Enterprise arrived in 2267. The ship was lost there, and its fate remained unknown until Kirk's crew came to investigate.
The Enterprise discovered that the Archon had been brought down from orbit by Landru, the computer that ruled Beta III's placid population, and that its crew had been absorbed into the planet's regimented society. The memory of the ship survived in local legend, and the natives mistook Kirk's landing party for Archons, the name they gave to the outsiders who had come before.