The loss of the starship Archon at Beta III in 2167, when the machine Landru pulled it from orbit and absorbed the surviving crew into its society.
The fate of the Archon is the reason for the Enterprise's visit to Beta III in the original series episode The Return of the Archons, set a hundred years after the ship vanished. The Archon had come to the planet and been dragged down from orbit by heat beams controlled by Landru, the computer that ruled it.
Many of the crew died. The others were absorbed into the Body, the placid, controlled society Landru maintained. Their arrival passed into local legend, and an underground of people immune to Landru's control awaited the return of the Archons to free them.
Kirk's landing party is taken for those long-expected visitors. With the Enterprise under the same attack, Kirk and Spock argue Landru into destroying itself, completing what the first crew could not.