The codex Astartes is a holy tome penned by our progenitor Guilliman roboute during the Heresy and the Scouring, some 10,000 years ago in my time. It was the distilled whole of human knowledge, philosophy, and tactics relating to the practice of war. It was his gift to his sons, and even now we continue to add to it, as new things are discovered, and new tactics developed.

Though few know the entirety of the text, all warriors know some.