Mordian Iron Guard
"Discipline is the soul of an army. It makes small numbers formidable; procures success to the weak, and esteem to all."
- – George Washington
The Mordian Iron Guard are an army of the Imperial Guard, hailing from the planet of Mordian. They are renowned throughout the Imperium for being right hard bastards, never backing down and prepared to die to the last man as necessary, as well as being from a planet with an adjective for a name (the Earth equivalent would be if Italy were named Italian).
TL;DR 18th-19th century Prussians mixed with WW1 Italian Arditi in space!
They are also renowned for wearing spiffy dress uniforms, although they're not as classy as their equivalents in the Praetorian Guard.
Tactics
There are Imperial Guard regiments that rely on using well-planned strategies and combined arms tactics to overcome their foes. Then there are those that don't rely so much on strategy so much as throwing hordes of guardsmen into the fray, although said guardsmen are often some of the most badass motherfuckers in the entire galaxy. Then there are the Mordians.
The Mordian Iron Guard technically belong in the second category, but are so far around the curve that they start veering back into the first category. The first thing to consider is that they hail from Mordia, a world that actually has two classifications; half Death World, half Hive World.
Normally these two types of worlds are mutually exclusive, since a planet where everything is trying to Kill you would put a lot of people off the concept of actually erecting massive towering cities on it. Not so for the Mordians. The planet spins at the same speed it revolves (tidal locked), so one side of the planet is constantly bleach-roasted by the Mordian sun, and the other side lives in zero degree dick-freezing perpetual night, much like the planet Mercury (except that Mercury isn’t actually tidally locked) . Additionally, the absence of strong winds on the planet creates a thick and oppressively dark atmosphere (which makes no sense, because temperature differentials that large should cause huge winds as the hot air rises and the cold air sinks -- living in the twilight zone would be like living in a permanent hurricane). The Hive Cities are built on the night part.
Thus every Mordian native is also a hiver, with years of experience ducking and killing inside narrow corridors where hive gangs exist on every level, and crime is rampant in the places where the sun-don't-shine (i.e. that entire side of the planet). Unsurprisingly, this has earned Mordian the dubious honor of having the highest suicide rate in the Imperium after the destruction of Nostramo. It got so bad that up until recently there was a notorious chaos uprising and the Mordians had to fight tooth and nail just to keep their semi-hospitable half of the planet from being overrun. Between that and the extremely strict rationing that leaves the planet on the verge of rebellion at all times, the Mordian Iron Guard has learned to practice iron hard discipline (hence their name) and constant regimental routine as part of their secondary role as military police. Nearly every citizen on Mordia has served in the planet's military, as it was the only way to maintain global discipline.
Consequently, Commissars find a tour on Mordia as rather boring, and a little intimidating as the Mordians themselves will penalize Commissars that falter in their duties. Again, absorb this for a moment. In almost every other part of the Imperium, Commissar shoots you for lack of discipline or cowardice. On Mordia, You shoot the Commissar for lack of discipline or cowardice. This is then reflected in their form of battle tactics. Rather than ride Chimeras or wear camouflage, Mordian guardsmen march into battle in brightly-coloured cloth uniforms and bearing equally flashy banners. They then form up into neat firing lines or squares and shoot at their enemies in disciplined volleys. They don't break formation, they don't take cover and they never, ever retreat. While this is incredibly ballsy even by Imperial Guard balls-of-steel standards, it is also incredibly stupid because on the battlefields of the 41st millennium there are oh-so-many ways to wipe out massed formations of infantry out in the open. These sorts of tactics were on their way out in the American Civil War (appropriate as Mordians look quite a bit like US Army from that era) when caplock minie ball rifles and rifled muzzle loading cannons began to appear. Even Space Marines will take cover whenever they can. But, somehow, the Mordians still survive their battles and keep going. Considering the horrors the Imperial Guard fights, you have to wonder how their training conditioned them so well. Then again, maybe you don't want to know.
Although a few sources note that this baffling lack of common sense causes their adversaries to underestimate them, it still doesn't really make up for employing infantry tactics that have long been redundant. That being said, the Mordians do have a few tricks up their sleeves, like flak and carapace armour actually sown into their rigid uniforms (which makes it both elegant AND protective), as well as having heavy armour companies with over a dozen Baneblades each. They also have 'veteran' companies who wear the incredibly-more-badass darker uniforms who do actually observe real infantry tactics and perform the scout work. Only War also mentions that each Infantry Squad receives a Leman Russ Battle Tank for support (actually, it doesn't, since it provides as its premade regiment the Mordian 12th Armored Regiment). Not a Command Squad. An Infantry Squad. That might be a line of men firing at you, but that line has a dozen or so tanks right behind it, firing all of their guns. Its expansion, Hammer of the Emperor allows for mixed regiments.
However, one must note that the fact that they're never taking cover means that they're not spending any time not aiming or not firing, meaning that relative to other armies, they could keep you pinned a whole lot more easily. So while they might die off really quickly, they'll do a hell of a lot before they drop. Since their armor has both flak and carapace armor woven into it, the Mordian Guardsmen are well protected against concussive force, shrapnel, and even against direct fire. So, not only is it a wall of lasers shooting at you, shooting the wall has about as much effect as spitting on a tank. Besides, they probably don't stand in a row, they probably have the first row go prone, the next row squat, the third row crouch, and the fourth row stand. Then they have four rows of lasguns firing in a concentrated barrage at the same target. Supported by a Leman Russ's battle cannon plus sponson and hull-mounted weapons and probably a pintle-mount. Then remember how long and deep the gun line is, given how numerous the Guard is. That isn't even accounting for man-portable mortar teams, vehicle-mounted mortars, basilisks, manticores, and the armored companies of over a dozen baneblades each.
That's scary and is the perfect tactic for the Guard because their reserves have reserves. It also happens to be the perfect tactic against Tyranids and Orks. The Tyranids are simply that numerous and the Orks are specifically stated to miss anything they aim at, so bunching up into a single target is the best defense against them. Maybe good against Necrons, too, since cover really won't protect you against their weapons. Firepower will.
The planet Mordian along with the entire Stygius Sector was invaded by Chaos Forces led by warbands loyal to Tzeentch. The Iron Guard held their ground as reinforcements arrive, but there were too many heretics and slowly the Imperial defenders are being bled dry. (Didn't help that the dead are being raised as zombies or that the Warp claimed several Imperial ships and an entire planet during the conflict.)
Further reinforcements came in as an Eldar Strikeforce from Craftworld Ulthwe aided the Imperium. Being the cryptic pricks they are, they tell the defenders that they can't win and advised them to leave the sector. (Seeing as the Chaos Invasion is led by the Thousand Sons under Magnus as well as M'kachen, they are kinda right). Fortunately for the Iron Guard, their resolve apparently let them last against a Demigod with twisting and changing ass-eating Daemons. That paired with the arrival of several groups of Space Marines allowed Mordia to hold out, and for the siege to be relieved.