Clouds of dust filled the Elysian Gate. An open volume of space three thousand kilometres across, it glittered with folds of fine particles. Hundreds, perhaps thousands of years of void-ships translating to the warp at this point had seeded it with drifts of the soft, grey matter that formed in the wake of a closing breach. The Jovian clans and the Navigator houses had a name for it. Siren Ash, they called it. There were tales, they said, of prospectors who had tried to harvest the dust and could desire nothing else once they’d touched it. True or not, the dust remained, slowly coiling through the volume of the Elysian Gate, like smoke caught in a glass orb.

The gate had always been guarded. Things had come out of it during Old Night, things that the Uranus Habitats remembered in tales of star-vampires and iron men. They had built the first fortresses around the gate, watching it with guns and warriors. These stations were called the Eyes of the Old God, and they had kept their vigil as the rest of the Solar System slid into the depths of the Age of Strife.

Then the Great Crusade had risen from Old Earth and brought the Uranus Habitats and moons into the nascent Imperium. The watchful stations had grown, their hereditary warrior clans augmented by Martian weaponry. Ships had begun to pass through the gate into the immaterium, and others had returned. The Navigator houses re-established their fiefs amongst Uranus’ twenty-seven moons, and the volume of space between the gas giant and the Elysian Gate had become an ever-glittering stream of light as ships looped from the warp to the profusion of orbiting habitats and void-stations.

Horus’ war had changed that. The flow of ships had become a sullen trickle, and the stations keeping the long watch had bloated with fresh armour and weaponry. Every foothold of humanity that could mount a macro cannon or host a fighter squadron had found itself made a fortress. Amongst this, facing the dark of the Elysian Gate, the ships of the Second Sphere hung motionless, crenellated and beweaponed, in the glittering abyss.

In the gate, the dust moved. A slow swirl gathered and coiled in on itself. Clouds hundreds of kilometres across flowed and folded. The dust began to spark. Tiny worms of lightning flicked between grey motes. The clouds began to glow, now green, now bruised violet, now bloodied ivory.

The waiting fleets of warships lit their drives. In their sanctums, astropaths began to weep. In the habitats and stations, the low howl of warning sirens woke millions from dreams of shadows swallowing the sun. On the bridge of the Monarch of Fire, Lord Castellan Halbract, commander of the Second Sphere of Sol’s defences, watched the reports blur across his sight as the helm of his Terminator armour locked in place.