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 sand 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 the Star Trek: Star Charts (pp. 60 & 66), Cheron was located in the 83 Leonis system in the Beta Quadrant. 83 Leonis was listed as a single star system with a K class primary with a magnitude lower than Sol. In the mid-22nd century, the 83 Leonis system was located in non-aligned space. In 2378, the system was located in or near Federation space.
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).