Night falls on the golden age of humanity.

Horus Lupercal, Warmaster and favoured son of the Emperor has plunged the Imperium of Mankind into a brutal civil war that will tear the galaxy asunder.

The atrocity of the Isstvan Drop Site Massacre has revealed the full extent of the Warmaster's treachery. Brother fights brother and the the Ruinstorm occludes the Emperor's light from much of the galaxy.

Roboute Guilliman, fearing the Emperor dead and the grand dream of the Imperium lost, has founded the Imperium Secundus in an effort to keep the flame of the Emperor's ambitions alive.

Sanguinius is its regent, the Lion is its Lord Protector.

Three full loyalist legions, along with chapters from many more, are cut off from Terra. But they do not stand idle.

Across the Imperium Secundus the beleaguered loyalists do battle with the traitor legions, attempting to secure the boundaries of their new Empire.

In the galactic west of their domain lies the Fornax sub-sector. Its lords, the techno-prelates of the Foraxian Hegemony, have refused to declare for either side. Paralysed with indecision they have chosen instead to build up their defences and hope to ride out the conflagration as the galaxy burns around them.

In doing so they have chosen their own destruction.

Both loyalist and traitor task forces approach the Fornax cluster, intent on securing its vital promethium rigs, refineries and prodigious industrial capacity for themselves.