For the impatient, here's the official synopsis:
In Halo: Cryptum, Greg Bear began a three-book arc set in the era of the Forerunners, the ancient and enigmatic creators and builders of the Halos, which continued in Halo: Primordium. Now, 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—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.