A weapon of mass destruction that could only be fired once, either for Earth's salvation or it's utter doom.
Circa late 1976 or early 1977, during the First Family's Ring of Worlds inter-dimensional adventure, an entity know only as Qestoth Sul informed Dr. Furst of a powerful weapon that was hidden on Earth millennia ago by the Firstmen upon their departure from Earth. It was to be fired only once, by an innocent against the greatest trial Humans would ever face.
Upon their return, they went to an island where among the ancient, perhaps even alien ruins, they discovered the Innocent Gun. Their first instincts were to return it to their lab to study it, reverse engineer it and perhaps discover its power source and how it worked. Fortunately, Simon Magus somehow sensing that the powerful device had been unearthed, appeared on the island to warn Dr. Furst of the weapon's purpose and potential danger. He advised that Dr. Furst keep it in a safe place for many now sought it, including The Demonwaves, evil creatures who where already approaching the island, lusting for the device to use as a weapon of ultimate destruction.
It has been kept safely at First Family's Headquarters, until 1982 when the Pyramid Organization stole it the night before the Silver Agent's big return.
This all led to the Rise of Kerresh the Devastator, which was one of the Silver Agent's crisis points. Taking charge, the Agent, had Honor Guard, the First Family and the Omega Rangers under his command. While he sent the Omega Rangers to free the Apollo Eleven who were recently captured by Pyramid. During the battle the Point Man discovered the Gun knowing it was the most powerful weapon in existence. Even with Grimoire warning him of the dangers, the Point Man fired the gun anyway.
While, it did save Earth and push The Kerresh form of The Incarnate out of existence, it also had unforseen consequences. It neutralized the Apollo Eleven and caused a major tear in reality where the gun had been fired. While the N-Forcer and Dr. Furst were able to contain the tear, there were hundreds that were killed because of the Point Man's actions of firing the gun at the wrong enemy at the wrong time.
Astro City: The Dark Age Book Two #3, 4
Astro City: The Dark Age Book Three #3, 4