The Omega Codex was a tome that was codified by the Space Wolves Legion following the Razing of Prospero in 004.M31, during the opening days of the Horus Heresy. This tome was a compilation of the VI Legion's experience, strategy and tactics gained in the fires of war, which gave explicit instruction on how to fight and defeat other Legiones Astartes should the Emperor ever decree their destruction.
This controversial tome was a source of great contention between the various Space Marine Legions, with many of them petitioning the Emperor for its destruction.
Following the censure of the Thousand Sons Legion for their violation of the Emperor's Decree Absolute which forbade the use of psychic abilities and sorcery, as agreed upon at the Council of Nikaea, the impossible had been brought into the realm of reality -- for on the soil of Prospero, Space Marine had fought Space Marine for the first time. The unthinkable had been given precedent, made real by the simple fact that what had happened once would occur again and any one of the remaining seventeen Legions might be the victim.
No little ire was directed towards the Space Wolves, for it was they who had shattered this unwritten taboo, no matter that it had been on the orders of the Emperor Himself. Moreover, of all the Legions, it was the Space Wolves alone who possessed any experience of real war against other Astartes, knowledge from which they quickly codified a body of tactics and strategies intended to aid in defeating other Legions should the Emperor ever decree their destruction.
This Omega Codex was a direct threat to the other Legions, and its contents were shared only with the Sons of Horus and the Warmaster Horus himself, an error Leman Russ would not realise until later. Almost every Legion petitioned the Emperor for the destruction of the Omega Codex and the censure of the Space Wolves for its creation, save the Night Lords, Alpha Legion and Iron Hands, who instead demanded it be shared with all of the Legions equally.
In the short solar months that stood between the Fall of Prospero and the start of the Horus Heresy, a number of the Legions shifted garrison forces and amended inter-Legion treaties to shore up their personal defences and exploit potential weaknesses should the same fate as the Thousand Sons befall them too. As with the larger Imperium, Prospero's fall saw the once famous unity of the Legiones Astartes torn asunder and where eighteen great Legions once stood united as a single undefeatable host, now seventeen individual and vulnerable armies waited uneasily for the next to fall.
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 69