Waning
The Waning is one of the major time periods of the Imperium of Man. It was preceded by the Age of Redemption and followed by the Time of Ending, stretching from the beginning of M39 to mid-M41.
So remember how the Age of Redemption had all that crusading and whatnot going on? As predicted (by rational people), this ended up being a bad idea. Basically, it was "Shit Falling Apart: Imperial Edition," as xenos took back everything they had lost (and more), Chaos cultists started rebelling like crazy, and the High Lords of Terra imposed ever-stricter rules to stave off total collapse, including turning over whole systems to Space Marine Chapters for direct governance.
And if that sounds bad, well... it's called the Waning, what the hell did you expect?