In 2372, the Myhr'an planet killer destroyed Starbase Expanse 4, appearing to kill thousands of people in the process. It was also assumed to be used on several Wthaure and other colonies, including a Trookanian colony the USS Marsh responded to a distress call from months earlier.
Shortly after SE4's destruction, the USS Cantabrian discovered a trail of thimaldicide gas, a by-product of bilitrium and an antimatter converter. Starfleet speculated the planet killer was powered by bilitrium, an incredible but extremely dangerous form of power. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part One")
The planet killer's weapon had eight, arm-like appendages at its aft. When in standby mode, the arms were closed on a central point to protect the weapon barrel. When activated, the arms opened, revealing the weapon barrel at the arms' bases, while the "hands" acted as focal guides. The weapon did not appear at full capacity during any attack against a Federation target.
In its final attempt at an attack, the Myhr'an planet killer prepared to fire on Canaileus Prime. The Starfleet response was successful in stopping the Myhr'an, and, with the build-up of energy in the weapon waiting to be fired, the planet killer exploded. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part Two")