A manufactured planet only a few thousand years old, built by the extinct Kalandans and still defended by lethal projections of its last commander in That Which Survives.
The Kalandan outpost is the puzzling world of the Original Series episode That Which Survives. It is roughly the size of Earth's Moon yet has the mass of Earth and a breathable atmosphere, and geology suggests it is no more than a few thousand years old. It was in fact built by the Kalandans as a supply base.
While making the planet, its builders accidentally created a disease that killed them all. The station's computer continues to defend it by sending out replicas of the last commander, Losira, each able to kill one chosen person by touch.
A power surge throws the Enterprise 990.7 light years away and maroons Kirk, McCoy, Sulu and the geologist D'Amato, who is killed. Kirk's party finds the computer room, and Spock arrives in time to destroy the machine.