Tarsus IV

Tarsus IV was the inhabited fourth planet of its star system. This was the home of an Earth colony.

History
In 2246, an exotic fungus destroyed most of the colony's food supply, leaving the colonists, of which there were eight thousand, in serious danger of starvation. Governor Kodos made a drastic decision: he ordered the deaths of four thousand colonists, so that the other four thousand might live, selecting those to be killed according to his personal theories of eugenics. Supply ships arrived earlier than expected, but too late to save the four thousand people Kodos murdered. All that Earth Forces found of Kodos was a burned body, and he was presumed dead, until his chance discovery on Planet Q twenty years later.

Among the survivors of the Tarsus IV incident were James T. Kirk, Kevin Riley, and Thomas Leighton, who were three of only nine survivors who knew Kodos' face, and who would be able to identify him. In 2266, they were the only three of the original nine eyewitnesses still alive. 

In 2328, Tarsus IV was the destination for passengers traveling from Gamma Trianguli VI via commercial transports. had traveled on the SS Manoa, Jack Litchfeld had traveled on the SS Kogin, and and  had traveled on the SS Wisconsin. (, okudagram)

Background information
In the final revised draft of the script for, the Tarsus IV massacre was referred to as "the rebellion on Tarsus IV". In the final version of the installment, though, no such rebellion is established as having taken place on the planet. In the same script, Kirk also described Tarsus IV as "a colony which was disintegrating before my eyes! Starvation! Rioting! Disaster!" The method of execution Kodos used was said to have been an antimatter chamber, which, after he threw a switch, completely disintegrated the four thousand colonists all at once.

In the final draft script of the episode, Quark told Benjamin Sisko that the Ferengi had never had an incident like what had happened on Tarsus IV. (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library)

According to Mike Sussman, a computer readout on the USS Defiant (NCC-1764) in stated that Hoshi Sato was among those killed by Kodos on Tarsus Four. However, the relevant section of the bio did not appear in the finished cut of the episode.

According to the Star Trek app accompanying, John Harrison's bio claimed he was one of nine survivors of a 2246 attack on the planet, his parents Richard and Sara perishing. It was not made explicitly clear if this "attack" was the same event that happened in the regular timeline.

According to the reference book Star Trek: Star Charts ("United Federation of Planets I"), in 2378, Tarsus IV was a Federation member.

Apocrypha
In the Shatnerverse novel Avenger, the famine was an act of eco-terrorism, the testbed for a plague created by the, a radical environmentalist group who believed that the "cosmic plan" that formed the basis of the Prime Directive should apply to planetary ecologies as well – after the famine, they assisted Kodos in his escape and provided him with the name Anton Karidian.

In the Enterprise Logs short story "Though Hell Should Bar the Way", the USS Enterprise (NCC-1701), under the command of Robert April, was en route to Tarsus IV with relief supplies when it was attacked by a Klingon vessel commanded by Kor – though the Enterprise was able to fight off the Klingon ship (and dishonor Kor by not destroying it), they made it to Tarsus only to find that the massacre had already taken place, leaving April haunted, wondering if, had it not been for the Klingons, he might have reached the planet in time to save the colonists.

The novel Drastic Measures, details the events of Tarsus IV. This takes place roughly 10 years prior to the Battle of the Binary Stars.