The Board is Set (Black Library Advent Calendar 2017 #23)

Malcador the Sigillite is troubled. The war fleets of the traitor Horus approach, and war on the Throneworld is inevitable. But with the Emperor on the Golden Throne, keeping the darkness beneath the Palace at bay, how can even the assembled might of three Space Marine Legions stand against the fury to come? Communing with the Emperor, Malcador plays out a wargame, musing on the key pieces and the unorthodox strategies that may be required to secure victory – and at what role he might play in the battles to come.

Read it Because
It's a startling insight into the relationship between the Emperor and the Sigillite, and shows just how aware they each are of the sacrifices that will be required to end the Heresy and bring peace to the Imperium.