Out of the hundreds of formerly inhabited worlds found by human explorers, more than fifty were home to alien civilizations that rose to their height within the past ten thousand years. Evidence suggests that many of these civilizations had learned faster-than-light travel and were in contact with one another, though none had ever reached our solar system. Despite their accomplishments, the Sigma Event struck them one at a time, leaving humanity as the sole inheritor of the galaxy. On many of these recently occupied worlds, explorers have found the same triangular symbol
Some structures marked with this symbol appear to be libraries, while others are laboratories or repositories for strange machinery. Many of these buildings contain diagrams and textual descriptions of a space-traveling object the inhabitants could not identify—a sphere adorned with thousands of long spikes, resembling a sea urchin. None of the facilities have photographic records of this object.
From other documentation found in these facilities, we know their inhabitants believed this object was responsible for the galaxy-wide extinctions we call the Sigma Event. The Xenohistory Corps refers to this alliance of interstellar researchers as the Sentinels, and refers to the object they were studying as Sigma.
Sentinel records explain why Earth has never detected the echoes of interstellar signals from these fallen worlds: they avoided sending long-range signals for fear of revealing themselves to Sigma. Since the planets holding Sentinel buildings are just as uninhabited as any other planet struck by the Sigma Event, it seems as if this precaution, and the other efforts of the Sentinels, were ultimately ineffective.
Still, the Corps hopes humanity can continue where the Sentinels left off, recommending that all travelers, satellites, telescopes, and colonists watch the skies for any glimpse of a spiked sphere moving between the stars.