A colony destroyed by the Gorn.
Cestus III is a planet that was settled by humans as a colony, and which had a Federation outpost on its surface. It was a small but important settlement which served as a base for research and trade, and for a time it was one of the more remote Federation holdings.
The colony's story is bound up with the Gorn, a reptilian species who had a territory nearby. The Federation had settled the planet, and the Gorn considered it to be part of their domain. They launched a surprise attack on the outpost, destroying it and killing most of its inhabitants.
The attack on Cestus III leads to the events of the original series episode Arena, in which the Enterprise pursues the Gorn ship and Kirk finds himself engaged in single combat with the Gorn captain on a nearby planet. Kirk is told by the Metrons that he must fight and win. The outcome of the episode, in which Kirk spares his foe, shows the Federation's commitment to mercy.