Trelane's planet killer

The planet killer was an automated Preserver starship, a doomsday machine in Q Continuum service in the 2370s decade. The vessel was conjured by Trelane to participate in the Q Conflict. 

Service history and disposition
In 2379, the four Starfleet ships selected as pawns in the non-corporeal power struggle were divided into two teams by Q to play capture the flag. To make matters more interesting, as the Starfleet crew would not battle one another, Trelane added a doomsday machine. Captain James T. Kirk of the USS Enterprise (NCC-1701) and his officers quickly briefed the other crews on their encounter with Decker's planet killer.

The planet killer almost destroyed the USS Defiant (prototype) before it was tractored out of its path by the Enterprise. USS Voyager fired phasers, which proved ineffective. Voyager and the USS Enterprise (NCC-1701-E) then switched to quantum torpedoes, which successfully breach the planet killer's hull of solid neutronium. The killing salvo was fired by the Enterprise-E, and the doomsday machine detonated. Thus, Trelane's pawns won this contest.