In the last years of the Forerunner empire, chaos rules. The Flood
- a horrifying shape-changing parasite - has arrived in force, aided by
unexpected allies. Internal strife within the ecumene has desperately weakened
Forerunner defences. Too little, too late, the legal rate of Juridicals is only now
investigating possible crimes by the Master Builder and others.
Evidence-gathering agents, known collectively as Catalog, have been
dispatched to collect testimony from the Librarian and both Didacts: the
Ur-Didact, treacherously abandoned in a Flood-infested system, and the
Bornstellar Didact, who accompanies the Librarian as she preserves specimens
against the dire possibility of Hal extermination. Facing the imminent collapse of their civilization, the Librarian
and Ur-Didact reveal what they know about the relationship between the
long-vanished Precursors and the Flood. The Precursors created many
technological species, including humanity and the Forerunners. But the roots
of the Flood may be found in an act of enormous barbarity, carried out beyond
our galaxy ten million years before. Because of that barbarism, a greater evil looms. Only the
Ur-Didact and the Librarian - husband and wife pushed into desperate conflict
- hold the keys to a solution. Facing the consequences of a mythic tragedy,
one of them must now commit the greatest atrocity of all time - to prevent an
insane evil from dominating the entire universe.