The Godsend activated
The Godsend was a weapon of mass destruction developed by the Terran Empire in the 23rd century. It triggered a chain reaction that passed from planet to planet like a virus, incinerating everything in its path, and was capable of devastating an entire quadrant.
Terran Emperor Philippa Georgiou had the Godsend created to ensure that, should her enemies ever succeed in assassinating her, they would be left with nothing to rule. The engineers who developed the weapon took their own lives after it was completed. The Godsend was presented to Georgiou by her personal slave and former lover San, who likewise committed suicide—apparently—immediately after as a gesture of defiance. Shaken, Georgiou ordered the device destroyed, but unbeknownst to her it was actually placed into storage at an Imperial weapons facility on Terra administered by Dada Noe. Most thereafter believed the Godsend to be merely legend.
In 2324, with the Terran Empire in crisis due to resource exhaustion, Noe absconded with the Godsend to the prime universe, intending to sell it and live off the proceeds. Section 31 learned of the deal, though not the identity of the weapon, and sent Alpha Team to intercept it at the Baraam. While they and Georgiou apprehended Noe, the Godsend was unexpectedly seized by San, who had faked his death. San planned to use the Godsend to throw the Federation into chaos, and in its wake lead a Terran invasion that would establish a new empire in the prime universe.
Alpha Team pursued the Godsend to San's ship in the Crescent Nebula, where the passageway to the mirror universe lay. Georgiou and Alok Sahar boarded the vessel and wrested the weapon from San, though its detonation sequence was initiated during the altercation. Georgiou used her biosignature to complete the activation and flew the ship into the anomaly, so as to collapse the passage between the two universes. Seconds before the explosion, she and Sahar were beamed away by Quasi and Lieutenant Rachel Garrett. (Star Trek: Section 31)