The game's development and sale by the publisher were discontinued in 2008, following financial difficulties at the parent company.

[2] The starter pack included several decks, rulesets for the opposing sides, and a variety of accessories and components.

The war is set in motion by Horus, previously Warmaster (supreme military commander) of the Imperium of Man and favorite of the Emperor.

[a] However the sets as a collection broadly cover the entire span of the campaign, from its earliest major battle to the final confrontation.

A single game of the Horus Heresy Base Set features two commanders and their respective fleets and armies struggling over control of a planet.

"Unaligned" cards, representing units or assets usable by either side, feature a set of scales.

The rarity is denoted by the number of small dots appearing at the bottom of the card, according to the following legend:[b] When the game was first introduced, the starter pack (Base Set) included two play decks, one for the Loyalist and one for the Traitor side.

Unaligned Horus Heresy cards can be used with either of the two, but not with new Dark Millennium types such as the "Ork" and "Eldar" decks.