‘If that is a single ship,’ Prayto said, ‘it travels with suspicious ease. The Blood Angels have been encountering it since they left Macragge.’

Guilliman stared at him. Prayto’s choice of words made pieces of the dark mosaic fall into place. ‘It travels,’ he repeated. ‘A pilgrim.’

‘You think it might be the cause of the constructs we’ve encountered?’ Prayto asked. ‘If so, can it be fought?’

‘Maybe not the cause,’ Guilliman said, thinking over the tales Sanguinius had described hearing in the refugee camp. ‘Perhaps a catalyst. The witnesses said its arrival marked the beginning of ruin. It brought change with it. They saw that. None of the survivors of Pyrrhan mentioned the fortress – that happened after they fled. After the Pilgrim had passed.’

‘I like that theory better than Titus’,’ Gorod said.

‘So do I,’ said Prayto. ‘We still don’t know what it is, or how to fight it.’

Or even if it’s here, Guilliman thought. Only the Blood Angels had had any true contact with it. It was a ghost, cruising through warp and minds. Its being was indefinite. It hovered between myth and threat. The Pilgrim was uncertainty itself, neither real nor illusion, lurking just beyond the horizon of the observable. From there it spread doubt.

Guilliman wondered if it had already targeted him. He pushed the question aside. It was another trap of uncertainty. He would not construct theoreticals on an absence.

A shadow. A phantom.

The bridge became an assembly of sentinels. The word went out across the fleets to watch for the Pilgrim. The scans were continuous. The guns of three Legions pointed at the depths of the surrounding tunnel walls. The hours passed. Guilliman stood at the pulpit, staring into the grey sepulchre. The stillness went on and on and on. There was nothing but the bones of life and hope and reason. The universe beyond ceased to exist. The temptation arrived, insidious as it was insistent, to believe that there never would be anything except the grave. The fleets had willingly entered the trap. There would never be anything to fight. The Legions would journey through the infinite grey until they too became still, became bone.

Quiet crept onto the bridge. The hum of the engines, the whine of servitor joints and the clatter of keys at data stations became a fading background. The voices of the mortal crewmembers as they queried and answered each other fell below murmurs. The quiet was grey. It was dust falling, burying light, burying hope.

Guilliman watched the bones. He did not waver. If this was the last thing he witnessed, then he would stand guard until death.