The symbol of the Imperium is an eagle with two heads. While there is more than one interpretation for the symbolism in that, it speaks plainly to a simple truth: that the Imperium is built not just on the glory of Earth, but on the might of Mars as well. While the blood and bone of the Great Crusade are the gene-forged bodies of the Adeptus Astartes, their armour, guns and chariots of steel and science are provided by the Forge Worlds ruled by the Martian Priesthood. While inscrutable and seemingly homogeneous to the eyes of outsiders, the politics and ideology of the tech-priests are actually ones of multitudes, complexity and conflict.

Horus Lupercal is of course intimately familiar with this and well aware of the fact that for his rebellion to have any hope of prevailing against his father's Imperium, he needs a hold on the Mechanicum as well. So like everywhere else, the shadow of the Heresy falls upon Mars even before the first shots on Isstvan are fired, and the loyalty of the Mechanicum will be tested between the old rule of the Omnissiah from Earth and the new future promised by the Warmaster from Cthonia…

The Mechanicum of Mars is one of the most iconic aspects of Warhammer 40.000. Both visually and philosophically, they are a defining aspect of the dystopian nature of the 41st Millenium. What better way to show that the promise of progress and understanding is forgotten than with the fact that science and technology are in the iron grip of a priesthood?

Consequently, the priests of Mars are also an ubiquitous feature of Imperial society in the times of the Great Crusade and the Heresy. You'll be hard pressed to find a Horus Heresy book without at least one tech-priest, magos, skitarii showing their red robes and mechadendrites. Compared to this, it's almost curious how few stories are actually about them. Only one novel has the Martian priesthood as its central focus, and short stories or novellas about them are also rather sparse.

As a consequence the Omnibus dedicated to them is the shortest of the whole collection. This doesn’t lessen the quality of the stories collected therein, though: the Mechanicum is a fascinating subject and the Schism of Mars is an event which is substantially important for the Heresy and the future of the Imperium as a whole. And quite frankly, there really just aren’t enough stories like this in the Heresy that take a dedicated look at cool factions and concepts that aren’t Primarchs and Space Marines.

This Omnibus is basically Mechanicum - Upgraded Edition. It collects Graham McNeill's novel Mechanicum with a novella and a handful of short-stories. All combined tell the tale of the Schism of the Martian Priesthood, their violent breaking into the Adeptus Mechanicus and the Dark Mechanicum as well as the resulting blockade of Mars through the Imperial Fists.

I originally planned to collect Mechanicum in an Omnibus together with Master of Mankind. While the latter is also about lots of other things, the Mechanicum is central to its plot and the events of the novel show an important step in the overarching story of the Martian priesthood. But when more and more short-stories flocked to these two and it became apparent that The Outcast Dead would have to fit somewhere in between, too, I decided to split the "Omnissiah"-Omnibus in two: Omnissiah I: Death of Innocence and Omnissiah II: End of Empires (s. Follow-up).

Omnissiah I ends with the short story Into Exile. A fitting title to end on for a chain of stories about a planet at war with itself. The events and main character from Into Exile segue directly into Master of Mankind. The true epilogue to Mars' story told across Mechanicum and Master of Mankind will be the story The Binary Succession, also collected in Omnissiah II.

[Primarchs] = FALSE

[Horus Heresy Characters] = FALSE

01000111 01101100 01101111 01110010 01111001 00100000 01110100 01101111 00100000 01110100 01101000 01100101 00100000 01001111 01101101 01101110 01101001 01110011 01110011 01101001 01100001 01101000

This doesn’t really have any prequels beside the opening omnibus Heresy Rising. I’d still advise to read a few Phase II Omnibuses (Shadow Crusade I, The Burning of Prospero, Shattersong, Shadow of the Warmaster I) before heading into Omnissiah I.

The three Omnibuses Omnissiah I, Scale and Stone and Omnissiah II form a loose "Solar System"-trilogy.

Omnissiah II: End of Empires is obviously the direct sequel to Omnissiah I and concludes the arc of the Mechanicum in the Heresy.

Scale and Stone is more loosely connected to the two, but complements Omnissiah II by showing in its climatic novel the second pre-Siege attack on Terra ("above ground" by the Alpha Legion in Scale and "below ground" by daemons in Omnissiah II). Reading all three completes the events on Terra and Mars pre-Siege and brings one mostly up-to-date with the status quo on the Birthworld directly before the Siege. In which order you read the two is mostly up to you, although I feel reading Scale and Stone before Omnissiah II has a more natural flow to it.

If you are eager to head into Omnissiah II and want to skip Scale and Stone for now, just make sure that you’ve read The Burning of Prospero before you start with Omnissiah II.

The following are recommendations from other series and the wider Black Library canon. If you are interested in the characters, factions or themes that were part this omnibus, the following books might also be to your liking.

Books from the 41st millenium:

• Forges of Mars-trilogy by Graham McNeill, starting with Priests of Mars

  • This trilogy of novels is about the journey of an Expeditionary Fleet of the Mechanicum, venturing forth to explore the depths of unknown space for the glory of Mars. The expeditionary fleet consists of a wide range of Imperial factions: tech-priests, skitarii, robots, Titans, Imperial Army, indentured slaves, even a company of Space Marines of the Black Templars, and has at its core sailing a massive ship straight from the Dark Age of Technology. The whole trilogy (plus a concluding short story) is available in the Forges of Mars-omnibus.

• Servants of the Machine-God by various authors

  • An anthology of twelve short stories about the forces and secrets of the Mechanicum. If you're eager for a cross section of what the Mechanicum is all about, this should serve you alright.

Videos (for additional video suggestions, visit the Additional References):

• The Xana Incursion Part I, the first of a two-part video series by Oculus Imperia on the mysterious forge world of Xana during the Horus Heresy: "Being the first in a pair of records pertaining to the Xana Incursion, with discussions upon the origins, paradoxes, and mysteries of the Xanian Mechanicum and their strange world."