Horus Heresy novel by John French, about the civil war between the Legions and the Dropsite Massacre on Isstvan V.

After Horus’s rebellion is revealed, Rogal Dorn and Malcador the Sigillite send a large force of Legionaries and Primarchs to suppress it. The loyalist forces are unaware of the extent of the treachery awaiting them, and end up victims of one of...