This expansion book for Warhammer: The Horus Heresy digs deep into the Garmon Cluster, a region of space that became the site of some of the largest pitched battles of the Age of Darkness. This book allows you to play out your own slice of this grand conflict, using an Onslaught Campaign system that links your games together as you fight through the Garmon Bulwark, as well as new Core Missions and narrative Apex Missions. You'll also find rules for Legiones Astartes Blackshields and Shattered Legions, new units for Solar Auxilia armies and rules for each Legion's attached Legiones Auxilia, as well as rules for notable characters from this tumultuous period.

Inside this 208-page hardback book, you'll find:

- The Crucible of a Dark Age – A full exploration of the final war for the Garmon Cluster, including a detailed timeline of the many battles leading up to that terrible clash, a closer look at the Blackshields and Shattered Legions, and details on the Solar Auxilia cohorts who fought alongside the Legiones Astartes.

- Campaigns of The Age of Darkness: The Garmon Bulwark – A full Onslaught Campaign system, providing rules for a linked series of games where each victory or defeat builds up to a final battle. Also included are specific rules to tailor your Onslaught Campaign to the struggle for the Garmon Cluster.

- Age of Darkness: Core Missions – Five Core Missions, balanced for both campaign games and Matched Play events. Race to claim vital strongpoints in Sudden Strike, adapt to shifting tides in Strike and Fade, or battle for the central ground in The Heart of Battle.

- The Garmon Bulwark: Apex Missions – Two new Apex Missions, with extensive special rules intended for campaigns and narrative games. Extract survivors from a mine-strewn hellscape in Across the Fields of Dead, or pit Primarch against Primarch in a Clash Atop the Ghal-Zammad.

- Legiones Astartes: Shattered Legions – Rules for assembling armies of mixed and mingled Space Marine Legions, whose behaviour is influenced in different ways by their clashing doctrines and hybrid tactics.

- Legiones Astartes: Blackshields – Rules for fielding armies and Allied Detachments of Space Marine outcasts, renegades, and splinter groups from various Legions, forged together into new and often strange warbands.

- Legiones Auxilia and Solar Auxilia – Four new units for the Solar Auxilia, including Hermes Light Sentinels and Aethon Sentinels, as well as a new Cohort Doctrine, plus rules for fielding Allied Detachments alongside Space Marines – including specific rules for every Legion.

- Legiones Astartes: New Units – Rules for five famous characters from the narrative, including Shadrak Meduson and Endryd Haar, as well as Command Squads and lower-ranked Optae officers for any Legion.

- Showcase – Along with atmospheric photography and annotated artwork, the book includes a showcase of beautifully painted miniatures for the White Scars, Blood Angels, Sons of Horus, Shattered Legions, and Blackshields.

In order to use the contents of this book, you will require the Warhammer: The Horus Heresy – The Age of Darkness Rulebook and one of the Army Books for Warhammer: The Horus Heresy, which are all available separately.