When you handed the rest of the Antikythera to Positron for study, you elected to hold onto this small fragment. It serves as a reminder of the events that news organizations have already dubbed...
The invasion began shortly after you left James Harvan's lab. Nemesis automatons, empowered by the energy stolen from Pandora's Box, descended upon City Hall in a bid to control the heart of Paragon City.
You and Positron planned out Paragon's defenses and moved to intercept the invaders, with some success. However, as time dragged on, more and more of Nemesis's forces were appearing in the city. It was clear something needed to be done to take the fight to Nemesis himself.
You returned to Harvan's lab with the Antikythera, hoping to find some way of using its connection to Pandora's Box against Nemesis. You discovered a way to use the device in his lab to 'empower' the Antikythera, enabling it to absorb any Pandora energy that wasn't otherwise contained.
Your efforts were just in time, as Nemesis's full-scale invasion had begun. He sent his greatest creation, the Nemesis Monstrosity, to bring Paragon's defenses to its knees. You made use of the Antikythera's new capabilities, and with each automaton you defeated, you gathered more of the power from Pandora's Box.
With your newfound strength, you faced down the Monstrosity, and destroyed it. However, upon doing so, a massive surge of power was absorbed by the Antikythera. It proved too much, and the Antikythera shattered, allowing all of the energy it had absorbed to dissipate. The mainstay of the Nemesis army, however, had been broken, and Paragon City was safe once again.