Krill led his people as they struggled to survive when the city builders took over. In the year 2266, he saw the Federation Starfleet starship USS Enterprise enter orbit of his homeworld. Krill expressed his thanks to its captain, James T. Kirk, and his landing party for destroying one of the machines.
However, Krill later became skeptical of Kirk's plans and saw him as a threat to his leadership. He tried to stop stop Kirk, but was subdued by his first officer, Commander Spock. Krill was then given the chance to lead his people against the machines, but rejected it in favor of going against them alone. The plan worked and Krill and his people were able to destroy the city builders. (TOS comic: "Invasion of the City Builders")