From Lexicanum

Age of the Imperium

Age of Rebirth - The Forging - Nova Terra Interregnum - Age of Apostasy - Age of Redemption - The Waning - Time of Ending

Overview

The Waning, was a period of time after the Redemption Crusades, where due to the exhaustion of the Imperium's military, world after world and system after system fell to Ork invasion, Chaos insurgence, or plain rebellion. Some systems were turned over to Space Marine chapters for direct government.^1-p125

Notable Events

• 995.M40 - 000.M41: The Macharian Crusades
• 000-070.M41: The Macharian Heresy
• 139-160.M41: The Gothic War
• 742.M41: The Damocles Crusade

Sources

¹ Warhammer 40,000 5th Edition Rulebook