- 100000 B.C.E.: the Forerunner-Flood War reaches a heightened state when a large portion of the galaxy succumbs to the Flood infestation.
- 100000 B.C.E.: the Forerunners create the seven Halo rings as a last resort to destroy the Flood.
- 100000 B.C.E.: monitors are assigned to maintain and safeguard each ring should they be needed again or used as a weapon by others.
- 01 Sedulous Keeper is assigned to Installation 01.
- 07 Fervent Delight is assigned to Installation 02.
- 049 Abject Testament is assigned to Installation 03.
- 343 Guilty Spark is assigned to Installation 04.
- 2401 Penitent Tangent is assigned to Installation 05.
- 16807 Blind Solecism is assigned to Installation 06.
- Monitor 117649 is assigned to Installation 07.
- 100000 B.C.E.: the Ark Portal is constructed on Earth by the Librarian.
- 100000 B.C.E.: Mendicant Bias engages Offensive Bias and the Ecumene at the Maginot Sphere.
- 100000 B.C.E.: the Didact activates the Halo Array at Installation 00, the Ark. The pulse kills as sentient life in the galaxy, excluding anyone sheltered within the Shield Worlds or on the Ark.
- 100000 B.C.E.: Mendicant Bias is defeated by Offensive Bias and taken to the Ark.
- 100000 B.C.E.: every sentient creature indexed on the Ark is returned to their home world per the Conservation Measure.
- 100000 B.C.E.: the Logician returns to Installation 01 where he'll conduct research and experiments on the Flood and technology to fight it.