Halo: Silentium (The Forerunner Saga #3 , Halo #11)

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 defenses. Only the Ur-Didact and the Librarian—a husband and wife pushed into desperate conflict—hold the keys to salvation. Facing the consequences of a mythic tragedy, one of them must now commit the greatest atrocity of all time—to prevent an unmatched evil from dominating the entire universe.