This 48-page softcover supplement contains:

The Fist of the Legions: Detailed background on some of the largest armoured assets available to the Legiones Astartes.

Mission Rules: The information you'll need to play the Vaults of Thrasis Legendary Mission, along with three Leviathan missions featuring asymmetrical objectives.

Additional Profiles and Liber Addenda: New Battlefield Assets, a Unit Profile for the Whirlwind Missile Tank, and rules for Legion Decurion upgrades for the Fellblade Super-heavy Battle Tank and the Falchion Super-heavy Tank Destroyer.

You will need a copy of Warhammer Horus Heresy: Age of Darkness Rulebook and either Liber Hereticus: Traitor Legiones Astartes Army Book or Liber Astartes: Loyalist Legiones Astartes Army Book to make full use of this supplement.