Armageddon Steel Legion
"If the tanks succeed, then victory follows."
- – Heinz Guderian
Take the Death Korps of Krieg, replace the doom and gloom with slightly less doom and gloom, throw them into the largest land war in modern Imperial history, and give each man his own
Chimera BEEEIIHHHNNBLLAAAAIIDDDEE. What you get is the Armageddon Steel Legion, one of the most heavily mechanized Imperial Guard units in the galaxy, with a penchant for fighting Orks. They're basically a mixed bag of the myriad forces that fought for the Axis in World War Two, probably primarily the mechanized/motorized encirclements attempted by the Germans that are known as Blitzkrieg. They wear bitchin' trench coats and gas masks, they fight Orks on a regular basis, and they have access to a metric fuckton of armored vehicles. They probably use Chimeras as condo[m]s.
The Steel Legion are not the only regiment on Armageddon however; there's the Armageddon Ork Hunters. Basically, they're a regiment of mini-Marbos, so good at fighting the Orks they can use Ork weapons; a damn-near impossible (or stupid depending on who you ask) feat considering that most Ork weapons either fall apart or backfire when used by a non-Ork. So they're all so badass they've become Orky in their own right, or they're, in fact, all Orks in disguise trolling other Orks and waiting for the opportune moment to krump the humies.
Essentially, if you want to play a regiment that looks as bitchin' as the Death Korps but with the ability to have some personality and tanks, the Steel Legion are for you. Unfortunately, despite being from a major warzone, they're only in metal. At least, from GW. Both Anvil Industry and Mad Robot Miniatures make affordable conversion bits that can be used to represent Steel Legion (Of course, if you are willing, painting Krieg minis mustard yellow looks a lot nicer)
The First War for Armageddon
So, minor bit of history. Armageddon is pretty much a big ass factory, that on three occasions has been trashed. The first time, Angron decided it would make for a nice summer home, and decided to party for a while with a couple thousand Bloodletters and fifty thousand Khorne Berserkers of the World Eaters. Much partying was to be had before someone called the cops with a noise complaint, and the Grey Knights, Space Wolves and whoever hadn't been killed managed to take back the planet, with ninety Grey Knights dying to banish Angron.
When the party was over, the Inquisition decided to take out the trash, meaning that they forced the entire population of Armageddon into labor camps and worked them to death on suspicion of HERESY, before repopulating the planet and pissing off Logan Grimnar and his altruistic Space Wolves to no end. This escalated into a catastrophically wasteful standoff between the Inquisition and the Space Wolves that ended with the Lord Inquisitor being chopped into an idiot salad. Baba Booey.
Armageddon 2: Electric Boogaloo
Jump ahead a few hundred years, and we get the real origins of the Steel Legion. Warboss Ghazghkull Mag Uruk Thraka thought that Armageddon would be a nice place for a WAAAGH and so there was a big ass fight. Commissar Yarrick led the defense of Hades Hive, holding out for months without support against a gigantic Ork offensive. This is where Yarrick first earned Ghazgull's respect; countering every attack plan the warboss had, and sending crackheads on suicidal charges into ork camps with explosives strapped to them.
With the Guardsmen stretched thin, the Blood Angels arrived to pull their asses out of the fire (See Deux Ex Machina), doing what Indrick Boreale couldn't and performing a successful STEHL REIN attack. The entire company dropped into the Ork Horde and fought nonstop until the Orks fled. Ghazghkull said he'd be back, so the Guardsmen began prepping for war.
Having lost the majority of its PDF and most of the off-world regiments, all that was left was charred corpses and scorched metal. As the spess mahreen apothecaries solemnly collected their fallen brothers' gene-seed, thousands of medicae could be seen retrieving countless standard issue adamantium balls, as well as the odd pair of power balls. Manly tears were shed on both sides and the repariation of the planets defences would begin.
The Third War for Armageddon
Preparing for his return, the Steel Legion became insanely fortified, with the Munitorium granting them a lessened tithe to keep local forces numerous enough to defend the planet. Millions of tanks, transports, and any other vehicle they could build were used in a gigantic buildup for the Orks return.
When they finally did, the Third War quickly escalated into the largest Ork WAAAAGH in history, drawing in dozens of Space Marine chapters, Imperial Guard regiments and even Titan Legions. After a mostly successful initial defense, the Steel Legion was slowly pushed back into heavy street fighting, turning the war from the Africa Campaign into Stalingrad, with tanks rolling off the factory floor directly into combat zones. The war mostly stopped, when Armageddon entered its "Season of Fire" (a major period of volcanic activity), so while the Orks managed to successfully capture almost half of hive-cities, the cities in which they were defeated became unreachable. Orks got bored, and withdrew most of their forces to space, though still a lot of them remained on Armageddon and continued to fight. Both sides declared this as their victory. Steel Legion units now use the permanent Ork infestation for training purposes, keeping them away from the remaining major hives.
When the Great Rift opened, the war started back up again with the return of Chaos to the planet. Currently, the Steel Legion is now stuck fighting not only the Orks, but also the forces of Khorne and Tzeentch.