Basically, the Imperium consolidated its strength, having finally rebuilt from the Horus Heresy.
The War of the Beast
The Imperium, after the heresy, enjoyed an odd period of relative peace, so long as we ignore silly and minor events like the Iron Cage world, the first Black Crusade, the first battle of Cadia, the Battle of the Fang, multiple traitor Primarchs fucking over many worlds, not to mention all of the Daemon cults that were popping up everywhere. The Adeptus Astartes were starting to die out as there were less and less campaigns to wage (despite the traitors still being around in huge numbers) and the forces of man consolidated their domain. Then appeared the Beast, the mightiest Warboss ever been conceived to ravage the galaxy. The Imperium was thrown to its last ropes and it took a lot of effort to survive. It bears special mention because its the turning point for the Imperium's organization: the former position of Lord Commander (basically a prime minister mixed in with general of the armies/navy) was abolished and the spirit of the Imperium went on from being relatively good and peaceful to start its decay into what would become the hell hole we know and love.
The Beheading occurred when a Grand Master of the Officio Assassinorum decided to annihilate the rest of the High Lords of Terra. Nigh one hundred years of competent governance ensued before the Grand Master the Space Marines stepped up and murdered him (albeit at great loss). Then nearly a century of anarchy consumed Terra before the Ultramarines gathered a group of successors and reappointed a set of High Lords. This also led to the creation of the Ordo Sicar(i)us, one of the minor Ordos of the Inquisition, which watches the Officio Assassinorum.
The Astropath Wars
The BRB gives precious little detail (none at all, in fact) about the Astropath Wars, but whatever they are, they sound Awesome.
Fantasy Flight Games' Deatwatch supplement Honour the Chapter mentions that a Chapter called the Carcharodon Astra (likely the predecessor to everybody's favorite Chapter of Space Sharks) took part in them. Sadly, it then mentions that the Astropath Wars have been so thoroughly scrubbed from Imperial records that it would be easier for a denizen of the 41st millennium to research the Great Crusade or Horus Heresy, which is saying something, given the denizens of the 41st millenium don't even know the Emperor was a super-atheist.
The Blade of Infinity
A pre-Heresy ship named the Blade of Infinity appears out of the Warp, broadcasting what appears to be a desperate warning. Oh, and it claims to have entered Warp space sometime around M10, long before the invention of the Warp drive. It disappears into the Warp moments before massive battlefleet of Chaos Space Marines appears.
The Year of the Ghosts
Another topic the BRB doesn't bother describing in great detail, it implies that a bunch of undead fought against Chaos. Again, this sounds Awesome, but will probably never be expanded on.
In an incredible display of grimderp (and the Black Templars' callous disregard for... anything, really), the Black Templars end the so-called Catelexis Heresy by executing the Cacodominus, an alien cyborg so psychically powerful it telepathically controlled thirteen hundred star systems (!). Sadly, its death scream echoes throughout the Warp, blowing up the minds of billions of psykers and obscuring the Astronomican, sending millions of ships to their death in the Warp. The BRB claims it is a steep cost for victory; NO REALLY?
Also, the 6th edition Codex: Space Marines puts forward a theory that the Black Templars don't have Librarians because they all got blown up by the Howling.
The War of the False Primarch
The War of the False Primarch was a dark and bloody episode of the Imperium's history, now largely lost to myth and purged from all records, that plunged the Segmentum Pacificus into anarchy from 780.M33 to 860.M33. The conflict was finally ended when the High Lords of Terra, conveyed the Pentarchy of Blood and tasked five loyal Chapters to destroy eleven others, that had been declared Traitoris Perdita for their actions during the war.