Slaughter Koorland
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 has get stuck in, you and your 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 an all, then suddenly, the world you are in starts breaking apart and up in the sky a huge Attack Moon appears, this is 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 to find out your fleet has been smashed by the orks. You endure in a piece of floating junk until you get rescued by a half-wacky techpriest by the merest hint of luck and you are brought back from the brink of death to discover you are the last Imperial Fist.
Ladies in 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 in legion strength (Problem Gorillaman?) to save Terra from a direct treat, 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 while stop Adeptus Mechanicus from secessioning.
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 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 crack army storms Ullanor a second time.
Despite the odds, Koorland manages to kill The Beast, only to find to everyone's horror, there are other 5 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 externals treats.