Rules of Engagement (Short 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.
Some years after the outbreak of the Heresy, Remus Ventanus, Captain of the Ultramarines 4th Company and hero of the Battle of Calth, takes part in the defense of Talassar against the invading Death Guard, while his warriors struggle to adapt to the new battlefield rules handed down by Roboute Guilliman. The Death Guard are lured into over-extending their lines and defeated, but several other worlds of Ultramar are lost. During a World Eaters invasion of Prandium, Ventanus is given theatre command and issues orders according to Guilliman's new doctrine which appear to be leading the Ultramarines to defeat before suddenly turning into a highly orchestrated trap that wipes out the World Eaters. On Quintarn Ventanus leads his company against Salamanders who have seized the capital city and manages to kill their leader just before he escapes. Later, the Ultramarines prepare for a last stand on Macragge following a successful invasion of Ultramar and Ventanus goes down fighting the Sons of Horus. Horus is revealed to be Guilliman in costume, and the entire sequence of battles live simulations played out by the Ultramarines in order to test Guilliman's new battlefield doctrine, the Codex Astartes.
- Roboute Guilliman - Primarch of the Ultramarines.
- Remus Ventanus - Captain of the 4th Company.
- Evexian - Captain of the 7th Company.
- Honoria - Captain of the 23rd Company.
- Urath - Captain of the 39th Company.
- Archo - Sergeant, 4th Company.
- Barkha - Sergeant, 4th Company.