Forging

The Forging is one of the major time periods of the Imperium of Man. It was preceded by the Time of Rebirth and followed by Nova Terra Interregnum, stretching from mid-M32 to mid-M34.

Basically, the Imperium consolidated its strength, having finally rebuilt from the Horus Heresy.

The Beheading

The Beheading occurred when a Grand Master of the Officio Assassinorum decided to annihilate the rest of the High Lords of Terra. One hundred years of anarchy ensued before the Space Marines stepped up and murdered him (albeit at great loss). 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.

The Howling

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.