It was brought about by a combination of factors: a slow unmonitored build-up of irritants and allergens in the Ship’s environment, and then the sudden emergence of a latent virus in a population already weakened. It was a multiple accident, impossible for the pharaoh designers of the Ship to plan away, for all their ingenuity. But given enough time - more than five thousand years now - such low-probability events inevitably occurred.

The surviving population crashed to the threshold of viability. For a few decades Rusel was forced to intervene, through booming commands, to ensure that the Ship was maintained at a base level, and that genetic-health protocols were observed and breeding matches planned even more carefully than usually.

The low numbers brought benefits, though. The Ship’s systems were now producing a large surplus of supplies, and there was no possibility of any more water empires. Rusel considered, in his glacial way, establishing a final population at a lower level than before.

It intrigued him that the occurrence of the low-probability plague mirrored the restructuring of his own mental processes. The day-to-day affairs of the Ship, and the clattering of the transient generations, barely distracted him now. Instead he became aware of slower pulses, deeper rhythms far beneath any transient’s horizon of awareness.

His perception of risk changed. His endless analysis of the Ship’s systems uncovered obscure failure modes: certain parameter combinations that could disrupt the governing software, interacting failures among the nano-machines that still laboured over the Ship’s fabric inside and out. Such failures were highly unlikely; he estimated the Ship might suffer significant damage once every ten thousand years or so. On Earth, whole civilisations had risen and fallen with greater alacrity than that. But he had to plan for such things, to prepare the Ship’s defences and recovery strategies. The plague, after all, was just such a low-risk event, but given enough time it had come about.

The transients’ behaviour, meanwhile, adjusted on its own timescales.

Once every decade or so the inhabitants of Diluc’s corridor-village would approach the shrine of the Elder, where the flickering Virtual still showed. One of them would dress up in a long robe and march behind a walking frame with exaggerated slowness, while the rest cowered. And then they would fall on a manikin and tear it to pieces. Rusel had watched such displays several times before he had realised what was going on: it was, of course, a ritualised re-enactment of his own last manifestation, the hobbling leader himself, the manikin poor overbright Hilin. Sometimes the bit of theatre would culminate in the flaying of a living human, which they must imagine he demanded; when such savage generations arose, Rusel would avert his cold gaze.