The "20 rules" refer to the 18 Legions plus the two Lost Legions (often homebrewed). A physical book forces you to scroll past the World Eaters to get to the Word Bearers.
While Games Workshop does provide digital versions of their books, many players find them lacking compared to community-driven efforts or industry standards: horus heresy 20 rules pdf better
Instant calculation of Shrouded saves or split-fire mechanics. Hold the Line / Overwatch The "20 rules" refer to the 18 Legions
To play the current version of Horus Heresy, you need specific resources beyond just the core rules. You can find official digital versions and updates on the Warhammer Community downloads page. Document Type Age of Darkness Rulebook horus heresy 20 rules pdf better