Armageddon Steel Legion Mod
It began around the time of Winter Assault, when players were largely nonplussed by how the Imperial Guard handled; it was neither as number-heavy nor vehicle-heavy as the Imperial Guard was want to be on the tabletop. Little of how the Guard was usually intended to play on the tabletop - with massed formations of mechanized infantry, backed with armored support and artillery, using various means to get forces behind enemy lines - was incorporated into the final version of the Imperial Guard, and this set a lot of players off - if they couldn't properly get the manliest faction down pat, then certainly, something had to be done.
With that in mind, the Mod team involved got their heads together and added a whole new faction via this mod - one that was wholly unlike the Imperial Guard that was released, but not being entirely divorced from it. The first major version of it was made available for Christmas of 2007, with the newest version being released in 2010.
It even included a mini-campaign all its own, specifically for the new faction.
The Good, the Bad, and the Ugly
In many ways, the Steel Legion plays very differently from Dawn of War's bog-standard Imperial Guard forces, as you have two very different routes to take through their tree, each of which grants you access to different units, upgrades, and even a different relic unit (both of which are variants of the super-heavy tank, the Baneblade). Unlike Firestorm Over Kronus/Purgation of Kaurava or Dawn of War Professional, it doesn't change the existing metagame or factions already in-place; instead it adds its faction and does its best to keep the Steel Legion in its own unique territory within the game itself. It does, unfortunately, change the scale of existing tank and transport units in the various factions (size-wise) to make them more-true-to-tabletop.
The dual-role nature of the Steel Legion means that you need to choose a specific doctrine to follow for the battle - either focused on Armor and vehicles, or focusing on mechanized infantry. Both offer unique upsides, units, and technology - and unlike a similar choice for the Tau in Soulstorm, the choice is not an end-of-tech-tree option without one simple choice (with all of the good upgrades available on one side) available for it. The faction plays very differently from the existing Imperial Guard, with its forces leveraged entirely differently, focusing on either grinding foes down in wars of attrition or in mobile combat that overruns enemy defenses.
Balance for the most part, is pretty good, though it's clear that the modders are none-too-pleased with how the game's developers religiously nerfed artillery. Some facets of the Steel Legion seem quite strong, though never quite as powerful as that which must never be mentioned. It seems to strike a delicate balance, and on that level, seems to have succeeded, though players more fond of the generalist approach of the original Imperial Guard may be put off by it, especially given how much stronger the Steel Legion is as far as options it has. The Steel Legion faction, as a whole, is a bit slow to get rolling, but can be extremely hard to stop if it gets up to speed, hitting the enemy with wave attacks and artillery/air strikes.