Horus has rebelled against the Emperor by purging his and his brother’s Legions, but what was supposed to be a secret act of murder has been revealed. 
In the Emperor’s name, Rogal Dorn and Malcador the Sigillite dispatch the largest force of legionaries and primarchs ever assembled to end Horus’ rebellion before it can begin. But the loyalists, on their way to enact vengeance, do not know the depth of the betrayal they face. They will not be the perpetrators of a massacre, but its victims.
The last hope of returning to the Great Crusade and the unity of the Imperium will die in the dust of Isstvan V, and all those that survive will live in a new age of darkness.