The Codex Astartes is a great and sacred tome of military organisation, strategy and tactics written by Roboute Guilliman, the primarch of the Ultramarines Space Marine Legion. It outlines Guilliman's ideal for the moral behaviour, order of battle and tactical doctrine of a Space Marine Chapter. Its primary purpose was to restructure the Imperium of Man's most potent military forces to prevent another civil war like the Horus Heresy.