The rise of digital platforms has also made it easier for fans to access Horus Heresy PDF materials. E-book stores and online libraries often host Horus Heresy novels and graphic novels, making it possible for fans to explore the lore in digital format.

The Horus Heresy began with the Ullanor Crusade, a massive campaign led by Horus to conquer and unify the galaxy under the Imperium's rule. However, as Horus grew closer to the Chaos Gods, he became increasingly disillusioned with the Emperor's rule. The final straw came when Horus was ordered to return to Terra, the capital planet of the Imperium, to attend a council. Believing this to be a trap, Horus rebelled, and his legions turned against the Emperor.

The Horus Heresy, named after the Warmaster Horus, was a civil war that shook the Imperium to its core. Once the most trusted of the Emperor's generals, Horus was appointed Warmaster, leader of the Space Marine Legions, the elite superhuman warriors who served as the Imperium's military might. However, Horus was gradually corrupted by the Chaos Gods, malevolent entities from the Warp, a realm of energy and consciousness that underlies the material universe.