From Warhammer 40k - Lexicanum
The Principia Belicosa was the standard organizational template of the Legiones Astartes devised by the Emperor during the Great Crusade. Based on the revered text of ancient Roma, it succeeded the Hexagrammaton formations of the early Astartes during the Unification Wars, and was succeeded after the Horus Heresy by the Codex Astartes.
- 1: Angels of Caliban (Novel) Chapter 3
- 2: Index Astartes I, Codex Astartes, pgs. 12–16
- 3: The Horus Heresy: Age of Darkness, pg. 30