It includes two stories that take place on Terra, one of which occurs long before the Heresy and adds to the background regarding the Imperial Truth; another entry in the compilation is a Primarch origin story, covering the contentious circumstances under which the gladiatorial Primarch Angron takes command of the 12th Space Marine Legion, which he renames from the "Warhounds" to the "World Eaters". However, in GW numbers, it’s a totally plausible number as they started to hype up this release starting in May 2022 with the Horus Heresy open!

The stories present various facets of the unfolding conflict, as suspicion, insecurity, and paranoia spread through the galaxy on the wake of the Warmaster's betrayal.Mechanicum is the first book in the series not to focus on either the Primarchs or their Space Marines Legions. Molech is eventually conquered with the aid of the treacherous Knight House Devine, which has pledged itself to Slaanesh. Included weapon options are a gravis bolt cannon, a gravis autocannon, a gravis melta cannon, and gravis lascannon. Hunter's Moon reveals the fate of the small pack of Space Wolves assigned by Malcador to guard Alpharius—or to slay him if he proved false.

The book adds more background concerning the battle at Istvaan V, at the same time showing the drama of a squad of his surviving sons and the shattered legions while they fight to retrieve an unknown artifact that will change the course of history in the coming days of the rebellion. This kit is packed with options and makes 20 Space Marines, so you can muster either two 10-man squads or one big 20-man one. The Serpent features a Davinite priest, Thoros, who overthrows a nascent Chaos cult, presumably on Davin itself in preparation for the coming age of darkness. The embittered Iron Warriors have aligned with Horus out of frustration with the ignominious and unheralded role assigned them by the Imperium; before the story begins, they lash out in unforgivable genocide. It has with quad-lascannons for the side sponsons, a choice of hull-mounted lascannons, heavy bolters, or heavy flamers, and a pintle-mounted havoc launcher, multi-melta, heavy bolter, and heavy flamer.

Knowing followers of the Chaos Gods, though rare, refer to their faith as the Primordial Truth, or the Primordial Annihilator.