A Horus Heresy short story

As the Horus Heresy reaches Ultramar, Captain Ventanus of the Ultramarines must test his primarch's tactical genius against the most terrible of foes: other Space Marines.

READ IT BECAUSE
The first glimmerings of the Codex Astartes take shape as the Ultramarines begin to practice tactics that can be adapted to fit any foe. And Guilliman puts pen to paper and writes two words that will change the shape of the Heresy...

THE STORY
Roboute Guilliman has put the finishing touches to his great work - the Codex Astartes. This tome will show his Ultramarines how to deal with any tactical situation, no matter the foe. But as the Horus Heresy reaches Ultramar, Captain Ventanus of the Troublesome Fourth must test the strictures of the Codex Astartes against the one foe that the primarch never expected: other Space Marines.

A short story from the Age of Darkness anthology.