Rules of Engagement

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.