A massacre of colonists on Tarsus IV.
Tarsus IV was an Earth colony that faced famine in 2246 when an exotic fungus destroyed most of its food supply. Governor Kodos, who believed in eugenics, decided which colonists would live and which would die, and ordered the execution of about four thousand of the eight thousand people there. Relief ships arrived soon afterwards, too late for those killed.
A burned body was found and Kodos was presumed dead. Only a handful of survivors had seen his face, including the young James T. Kirk, Kevin Riley and Thomas Leighton.
In the original series episode The Conscience of the King, set in 2266, Leighton tells Kirk that the actor Anton Karidian is Kodos. Leighton is murdered soon afterwards, and other witnesses die one by one. The killer turns out to be Karidian's daughter Lenore, who has been killing the witnesses to protect her father. When she tries to kill Kirk, Karidian steps into the line of fire and is killed.