The Harrowing (The Horus Heresy #Short Story)

The garrison world of Callistra Mundi is threatened by traitor forces – none other than the insidious Alpha Legion. Like so many before it, this attack will not come as a traditional legionary assault, but as a Harrowing. Operatives and turncoats are revealed within the Mechanicum ark freighter Omnissiax and her attendant battlegroup, and confusion will reign amongst the loyalist defenders. And only when none can tell friend from foe will the true destruction begin... Read it because
The Alpha Legion assault a garrison world in their own inimitable style, turning friend against friend until no one knows who is responsible... Jump in and see if you can figure out the mystery behind the Harrowing.