From the back of the book:
The First Dark Legion War has engulfed the entire solar system, with humanity hard-pressed to hold back the swelling tide of Darkness. The endless hordes of the Dark Legion march forth, stretching the Brotherhood and megacorporations to breaking point. During these dark times, desperation and ominous whispers drive some to the brink of Heresy in search for an advantage against the overwhelming Darkness. The search for long-buried secrets leads into a dark abyss.
The Purging of the Crucible is recommended for experienced players. Set during the events of the First Dark Legion War as the power of the sinister ISC branch known as Blackthorne grows, suggestions are provided on how run the adventure in different timelines.