: The elite mortal spear-tip of the Imperial Army. Uniquely, they are the only faction in this book that can be fielded by both Loyalist and Traitor players.

"Liber Imperium" is a Latin phrase that translates to "Book of the Empire" or "Book of Dominion". Without more context, it's challenging to pinpoint a specific PDF you're referring to. However, I can offer some possible leads:

While specific Legions often receive their own "Red Books," Liber Imperium consolidates the core rules for the Space Marine Legions. It acts as a foundational text for Crusade Era warfare, covering the wargear, rites of war, and unique units available to the Loyalist factions.

: This free PDF includes rules for older or "legacy" models not featured in the main Liber books, ensuring players can still use their classic miniatures in the latest edition. Factions Included

This is a massive, fan-made compilation document designed to bridge and unify rules across Fantasy Flight Games' older d100 tabletop roleplaying games, such as Dark Heresy Rogue Trader Where to find it:

The Emperor's personal bodyguards and elite warriors.