The badass precursors to the Imperial Guard, they are essentially an entire army of 30k Stormtroopers. When the Emperor finally got over his parental pride and realized that a crusade dedicated to helping humanity should probably have some actual humans in it, he got on with the Imperial Army, drawn from everywhere the Imperium had influence, the top 20% of whom were the Solar Auxilia. If you want to learn more about fielding these guys, you can take a leisurely stroll over to the tactica.
The fundamental level of an Auxilia force is a Tercio, so called for being composed of up to three sections (squads), and three different types of squads are available to them. This can lead to a lot of fun as one basic Lasrifle Tercio can throw down 120 shots as 36" Heavy 2 Lasrifles to literally melt one side of the battlefield in a hail of energy fire. On the other hand, Veteran Veletaris Tercios usually consist of three squads of ten veteran troops equipped with short-range weaponry such as the Volkite Charger or Flamers (or even power axes). Furthermore, with Tercios counting as one troops choice in the FOC, the elite few of the Solar Auxilia can outnumber even the horde armies of the Imperial Militia.
Being 30k's Stormtroopers, they have lots of nifty tricks and items, like alternate lasgun power settings, enhanced snapshots, hold-my-hands-for-WS4 formations and damn good morale (rerollable Ld10!) with the right conditions. Oh, this being 30k they lack 40k Stormtrooper's weakness in the vehicular department, as the Solar Auxilia can bring a wide assortment of Super-Heavies as mere Heavy Support choices, and have access to a tank commander that can ride said Super-heavies. Those were the times...
As it stands, they only have
one two characters, "Lord Marshal Ireton MaSaad", a retired badass who wins through sheer old man strength and comes with the best colour scheme (Warplock Bronze and green) to date, but is a little lacking in the rules department; and Surgeon-Primus Aevos Jovan, the mortal version of Fabulous Bill but loyal, interestingly spared from execution by the intervention of MaSaad himself. Not to fret, there is a wealth of options to customize a generic leader for whatever role, even able to go toe-to-toe with the champions of the Space Marines, although a Lord Marshall is arguably better served as a force multiplier with a command squad to push this train into the station.
Martians, With Death Rays
The original Core of the Auxilia were taken, along with their name, from the Solar System, including the red planet. More importantly, the Volkite weaponry favored by the early legions found its way into the hands of the Veletaris Storm Sections, the elite of the elite meaning that you can finally play out your Mars Attacks! fantasies on the tabletop, (admittedly without a weakness for Noise Marines).
Their distinct steampunk-esque getup, the Solar Pattern Void Armour, is a fully enclosed environmental suit that covers their needs of void warfare and combat on new and hostile worlds, providing protection without sacrificing fashion sense. On the tabletop, this translates to a 4+ save, with the Void-Hardened aspect granting a re-roll to damage from blasts and templates, which isn't too shabby if you're building them as a shooty horde.
Like the Guard of 40k, the Auxilia also make use of Abhumans, and are notably more mean to them. Ogryn Charonites were pretty much what happens if you cross an Eversor Assassin, Edward Scissorhands and a bunch of conscripted Ogryn, leaving your enemy facing a squad of remote-control, drug-filled Murder-Cocktails with the power-tool section of B&Q where their hands should be. With little space left in their pained minds for anything other than rage, they tend to go off on explosive, albeit controlled, rampages once their handlers flip the switch and plunge their minds into an aggressive 80's movie drug montage loop for the remainder of the battle/their lives.
The Auxilia are in a bit of a weird place with their vehicles as they don't have access to the cool gadgets of the Mechanicum, or the special snowflake treatment of the Legiones Astartes, they do however have one of the best transports in the form of the Dracosan, which can even take a demolisher cannon(!?!), at the expense of ten transport capacity. Suck it Rhino.
Recently, as of the fourth Horus Heresy book, one sculptor named Edgar Skomorowski over at Forge World has become tired of the multiple flavours of power armour he has had to work with thus far and decided that battles in the Age of Darkness could probably do with a dash of variety. Not only he alone made all models in range, which is already quite a feat, but also masterminded all the rules, resulting in hilariously powerful (some may say overpowered) army. Now you too can spend £70 for 100 points of your army! However, while the pay gap is colossal, the Auxilia are one of the more rewarding armies to play, with your granddadie's Kalibrav V-1 pattern 36" Lasrifle (things were better in his day), access to some of the better dress sense and LORD OF ALL HUEG TANKS, they can make a list tailored to their player for maximum fun. On the downside, that fun is done in full knowledge of the horrors awaiting you once your mortal life is forfeit and the deamons you bargained with to get those models come for your soul. Morbity aside, your army will be the envy of everyone you know, so it's swings and roundabouts really.
Bad news? Edgar Skomorowski no longer works at Forgeworld, and Solar Auxilia is in hands of Israel Gonzalez, who apparently don't want to continue Edgar's work, so it's unlikely Solar Auxilia will ever recieve new units.