Cheron was a formerly inhabited planet. This was the homeworld of the Cheron natives, an extinct warp-capable humanoid species that destroyed itself in internecine warfare. The planet was located in the southern region of the Milky Way Galaxy, near the Coalsack Nebula.
But in the course of that year, when the USS Enterprise scanned the planet, its instruments revealed large uninhabited cities, extensive traffic systems barren of traffic, and lower animal lifeforms and vegetation encroaching on the cities. There was no sign of sapient life, and there were vast numbers of unburied corpses in the cites. The civilization had evidently destroyed itself in a war fueled by racism. The Enterprise left the last two surviving members of the species, Bele and Lokai, behind on this world, to which Bele had forced Kirk to return him and Lokai. (TOS: "Let That Be Your Last Battlefield")
Background information Edit
"CHE-ron" was the pronunciation for the name of this planet, according to the script pronunciation guide for "Blood Oath".  Other than the guide, however, Cheron was not mentioned elsewhere in the script.
According to Star Trek: Star Charts (p. 66) and Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", pp. 32-33, 39; "Federation Historical Highlights, 2161-2385"), there was a star system named Cheron (Omicron Gruis), in which this planet might possibly have been located, in the Beta Quadrant. This was either a single star system with a F-class star (Star Trek: Star Charts) or a binary system with a pair of F-class stars (Stellar Cartography: The Starfleet Reference Library). In 2160, the Battle of Cheron, the final battle of the Earth-Romulan War, was fought in this system. According to a map of the Romulan Empire – prepared by the Stellar Archivist Lsel Essik, from the Romulan Master Data Catalog, in 2366 – Cheron was identified as a Romulan system. In 2160, the Battle of Cheron, the first battle of the Earth-Romulan War, was fought in this system. (In what possibly might be a mistake, Star Trek: Star Charts (p. 60) identified the 83 Leonis system as Cheron.)
According to the novels The Buried Age, Kobayashi Maru, and To Brave the Storm, the planet Cheron was the fifth planet in the 83 Leonis B star system, and was the site of the Battle of Cheron in 2160. Cheron was a strategically important planet for the Romulan Star Empire and was occupied with a military outpost at the time. Cheron was also where a Romulan fleet of 81 ships massed in preparations for an all out attack on Earth. The fleet was defeated and forced to retreat by allied starships. After the war, Cheron remained on the Federation side of the Romulan Neutral Zone.
In the novel No Time Like the Past, the Enterprise briefly returns to Cheron, with scans revealing that Bele and Lokai have now set up bases for themselves on different sides of the planet; a time-displaced Seven of Nine notes that, even in her era, the Federation avoids Cheron due to the unpredictable power and life-span of the surviving natives. The novel also reveals that the Cherons were destroyed when an attempt by Bele's side to create a virus that would destroy only their enemies resulted in the virus being released before it was ready during an enemy assault, with the result that both sides were destroyed (although Kirk and Seven, temporarily trapped in the past, were unaffected by the virus).