The horns blared a challenge. Across the airless void, they made their terrible, warp-created sound. It travelled the millions of miles that separated the fleet from the fortress. Perhaps the cry had come as soon as the fleets had been detected and it was only now reaching the ships. Perhaps the horns had only sounded on the instant. The laws of reality had been suspended in this star system, and the Lion knew that what mattered was not how the horns cried, but that they did. The sound smashed into the Invincible Reason, shaking the hull. It boomed through the bridge. It was deep, as deep as the heartbeat of mountains. And it was a wail, a rising, shrieking, ­raging wail that harrowed the soul. The Lion winced. He made himself breathe through the blast. He leaned into it. On the bridge below, officers screamed, blood running from their ears and eyes. Servitors collapsed, spines ground to powder. The electrical systems of the ship surged and stuttered, blowing out pict screens, setting control stations ablaze. The thrum of the battleship’s engines became a hammering roar, yet the cry of the horns sounded above everything.

There was a pause, as if a behemoth of Caliban’s myths were drawing a breath, and then the horns sounded again. The cry was more than sound. It cut into the port flank of the formations, culling the weak like a scythe. It pulled the Dark Angels frigate Undaunted and the cruiser Unsheathed of the Ultramarines away from the fleets. The ships, miles long, powerful enough to turn worlds to glass, tumbled like leaves in a storm, massiveness made minuscule. The nearest horn sucked them in, hauling them away faster and faster, until they were streaking at a small fraction of the speed of light towards the fortification. They crossed the event horizon of the war-horn’s cone and vanished into the darkness within. There was no explosion, no flowering of ignited plasma. There was the brutal severing of communication the moment the vessels entered the cone, and that was all.

The fleets continued their turn, still picking up speed, the angles of approach changing as slowly as the erosion of monuments. At last the Invincible Reason was parallel to the wall, and then at last its speed became the strength it needed to pull away. The collision alarms fell silent.

The Blood Angels cruiser Excelsis did not complete the manoeuvre. The curve of its turn was too long. It came too close to the fortress and fell against it. It did scar the void with its end. It became a long streak of flame against the fortifications. The death flare was brief and barely noticeable against the infinite expanse of the wall.

‘Brothers,’ Sanguinius said as the combined fleets began to increase their distance from the fortress, ‘can any of you return to the warp?’