At the time of the discovery of an extragalactic "planet killer" in 2267, Kirk theorized that the robot weapon was, in fact, that: "a doomsday machine that somebody used in a war uncounted years ago. They don't exist anymore, but the machine is still destroying."
Following the destruction of the "planet killer", Kirk noted the irony of its destruction, stating: "Way back in the 20th century, the H-bomb was the ultimate weapon, their doomsday machine, and we used something like it to destroy another doomsday machine. Probably the first time such a weapon has ever been used for constructive purposes." (TOS: "The Doomsday Machine")
The captain of the Orion vessel caught by the USS Enterprise breaking the Orion stance of neutrality in 2269 suggested converting the explosive minerals found in the surrounding asteroids into a doomsday bomb capable of destroying both ships. The plan, however, failed, and the Orion crew was apprehended and taken into custody. (TAS: "The Pirates of Orion")
The misinformed and paranoid Klingon commander Kruge believed that Project Genesis was the Federation's attempt at creating an ultimate weapon, and described the Genesis Device as a doomsday weapon. (Star Trek III: The Search for Spock)