|This article contains spoilers! You have been warned.|
"The only thing necessary for the triumph of evil is for good men to do nothing."
- – Edmund Burke
Slaughter Koorland, captain of the Daylight company of the Imperial Fists was one of the major characters involved in the The War of The Beast and, for some time, the last Imperial Fist. As you may imagine, this guy is a total badass and you should go read the series.
Just imagine for a moment the situation he gets stuck in: He and his brothers are deployed in chapter strength to eradicate a verminous xenos infestation in a backwater planet so the High Lords could feel good about Segmentum Solar being quite safe and all, then suddenly the world he is on starts breaking apart and up in the sky a huge Attack Moon appears (this being an entire planet looted by the Orks working as a portal to unload unending waves of greenskins.)
After some epic fighting he manages to get teleported into orbit just in time to find out the Imperial fleet has been smashed by the Orks. He endures in a radioactive piece of floating junk until he gets rescued by a half-wacky tech-priest by the merest hint of luck and he is brought back from the brink of death to discover he is the last Imperial Fist alive.
Ladies and gentlemen, what does our hero do?
Instead of falling to despair and joining Chaos like certain failures did at the first sight of problems, Koorland mans up and activates The Last Wall, a protocol long ago established by Rogal Dorn himself to restore the Imperial Fists to legion strength (problem, gorillaman?) to save Terra from a direct threat. This man, a mere Space Marine captain, manages to put in line chapter masters, comes back to Terra with thousands of Astartes unified under his command, disables an Attack Moon hanging over the Imperium's Throneworld, and then takes command of the Imperium at gunpoint, managing to make the High Lords of Terra slightly less incompetent and meanwhile stop the Adeptus Mechanicus from seceding.
The new Lord Commander of the Imperium goes to find Vulkan as he knows he needs a Primarch to beat the Orks back (humble
yet and rational, if you think about it), Koorland finds him and convinces him to assault Ullanor at the head of a combined imperial force. Sadly the imperials are defeated and have to pull back, but Koorland doesn't give up and comes back for a second round, but before that he executes a traitor Ecclesiarch as That Guy was asking to capitulate to the Orks and start worshiping them.
Sans-Primarch but with some Sisters of Silence and the newly created Deathwatch (yes, this guy created the Deathwatch, how cool is that?) he and an elite army of crack xenos killers storm Ullanor a second time. Despite the odds, Koorland manages to kill The Beast, only to find, to everyone's horror, there are another five Beasts, and dies while facing the second one; still, he is totally worth the praise, and his followers kept going. His story shows why the Imperium has endured despite all the incompetence, infighting, and external threats.
About 9% of this was written by Dan Abnett which explains all the awesome.