Aerius is a Hive World of the Imperium, a mighty industrial world, and keystone to the Imperium's control of its sector. During the 38th Millennium it suffered a devastating plague. In the early 41st Millennium, Aerius was once again smitten by a deadly plague, and as a result, millions died. There had indeed been many strange portents. A great comet had appeared in the skies of Aerius, the baleful star of legend, which appears only once in every two millennia, and whose appearance always presaged doom. Showers of falling stars descended on the world at the moment of its appearance. The plague was connected with the mysterious Black Pyramid, an impenetrable Eldar psychic construct which was built, unknown to the local human population, to restrain a Greater Daemon of Nurgle named Botchulaz