The Perishing Star is meteorite that struck the Earth in an event called Nightfall, destroying the old world order and ushering in a new post-apocalyptic era. The Mirror System records that the Perishing Star was previously an unrecorded celestial body until it appeared suddenly and crashed through the atmosphere, and Dreya says that its approach was unable to be tracked. However, the Year One: Perishing Star Crimebrand, by stating that it struck earlier than expected, seems to imply that at least to someone it was a known quantity before its impact. In any case, the meteorite landed on the "western continent" in a desert that became WhiteSands and obliterated the city of Maria.
The Perishing Star's impact crater was filled with water and became the DisSea. In a matter of weeks, the surrounding region transformed into a wasteland referred to by contemporaries as the "forbidden zone" or "death zone" -- the world's first Black Ring. In N.F. 23, the Keylan Expedition entered the forbidden zone in order to study the catastrophe, with the ultimate goal of reaching the Perishing Star itself.
The meteorite was made of an alien material heavier and denser than any known element. As Nightfall was the origin of Mania in the post-apocalyptic world, the Perishing Star itself may be the mysterious Source that is sought by several factions in DisCity, especially the Underground.