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.
But that's the pessimist's view! Tons of sweet victories for the Imperium also occurred.
Gothic War
Also called the 12th Black Crusade, Failbaddon launched a massive invasion of the Gothic Sector, largely to capture a couple Blackstone Fortresses.
This Black Crusade in particular is famous as the setting for Battlefleet Gothic.
Macharian Conquests
Lord Solar Macharius, a tactical genius that puts Creed to shame, pushes the boundaries of Imperial space the farthest they had been since the Great Crusade. In only seven years.
Yeah, Macharius is pretty boss. He even has his own super-heavy tank!
Macharian Heresy
When Macharius died, it was sad for most of the Imperium, but utterly disastrous for his territories. They collapsed into insurgency, civil war, and other general nastiness. It took seventy years and multiple Space Marine Chapters to end.