In 2246, an exotic fungus destroyed most of the colony's food supply, and its inhabitants, of which there were eight thousand, faced starvation. Kodos, implementing his own theories of eugenics, selected four thousand of the colony's residents to be put to death, so that the remaining four thousand might survive on the limited food supplies available. Among the dead were Kevin Riley's parents. Among the survivors were Riley, James T. Kirk, Thomas Leighton, D. Eames, and E. Molson. Of the survivors, these five and four others were eyewitnesses who could identify Kodos. At that time, Kodos had red hair with a receding hairline, a red mustache, and a beard.
In 2266, Thomas Leighton was living on Planet Q when a troupe of itinerant actors, the Karidian Company of Players, performed there. He became convinced that their star, Anton Karidian, was actually Kodos. Diverting the USS Enterprise on a pretense, he argued this case to Kirk, who was less certain. Kirk's subsequent research raised some issues, and when Leighton was found dead outside his home, Kirk's suspicions became stronger. He arranged for the Karidian Company to embark upon the Enterprise, so that he could study the matter more thoroughly. Attempts on the lives of Kevin Riley and Kirk himself all but confirmed Kodos' identity.
The Karidian Players were performing Hamlet, the price of their passage (so to speak), when Kevin Riley sneaked backstage. He had overheard Doctor McCoy making a medical log entry, and now also knew that Kodos might be aboard – and might be the person who had poisoned him and murdered his family. Riley stole a phaser and sneaked backstage, but was stopped from murdering Kodos in revenge by Kirk, who convinced him to give up the phaser.
Karidian overheard their whispers, and during a conversation about them, discovered that his daughter, Lenore, had been slowly murdering the witnesses who could identify him. Distraught that Lenore, the one thing in his life untouched by what he'd done, was, in fact, poisoned by it, he raced after her when she snatched Harrison's phaser and ran onstage. When he realized she planned to kill Kirk, he stepped in front of the captain, and the shot meant for Kirk instead killed Kodos. (TOS: "The Conscience of the King")
Kodos was portrayed by actor Arnold Moss.
In the final revised draft of the script for "The Conscience of the King", Kodos (according to Kirk) appointed himself as a Messiah, ordered Kirk left alive, and threw a switch which activated an antimatter chamber that killed the four thousand colonists all at once.
Although not seen on-screen, part of Hoshi Sato's bio from ENT: "In a Mirror, Darkly, Part II" – which was written entirely by episode writer Mike Sussman – revealed that Hoshi and her family were among the four thousand people who Kodos killed on Tarsus IV in 2246.
According to the novel The Autobiography of James T. Kirk, his full name is Arnold Kodos. However, the novel Drastic Measures calls him Adrian Kodos. This novel tells in greater detail the story of his actions on Tarsus IV and their aftermath, as well as how he evaded capture and faked his death at the time.