Thus Did The Imperium of Mankind Fight There

Informational: Cthonian Auxilia

History

Cthonia was brought into the Imperium early in the Great Crusade - in fact, it was one of the first - as it lay in one of the closest star systems to Terra. So close was Cthonia and its inhabitants of such hardy stock, that before the Great Crusade began or even before Mars was brought into the Imperial fold, Cthonians were already being brought to Terra via raids of the ancient mining world to become some of the first humans outside the Sol System to become Astartes. 

The majority of these Cthonian Marines would later become part of the 16th Legion led by Horus Lupercali. This legion, later known as the Lunar Templars, earned its moniker due to its crucial role in quelling the Lunar Insurrection, the very first legion to earn a moniker.

It is perhaps unsurprising then, that Horus would select Cthonia as the first world outside of Sol that he would bring into compliance, with the Emperor later officializing Cthonia as a Recruitment World for the 16th. 

Of course - other than Astartes - the world became a verdant ground for Exertus Imperialis recruits, for not all human males possess the aptitude necessary to accept the augmentations of a Space Marine.  

Cthonian culture also placed little regard for gender differences. Before Imperial Compliance, Cthonia was a world ruled by gangs who cared naught if their fighters or victims were men or women, boys or girls. 

You fought or you died regardless. 

Due to this factor, and that many of the men who would normally join the Auxilia on a regular Imperial world join the 16th Legion on Cthonia, a large percentage of Auxilia recruits are female here. 

Doctrine

Cthonia possessed no true nation-states before Compliance, only constantly warring gangs who knew nothing of modern war. This meant that Cthonian Regiments were taught everything from how to properly salute an officer to combined arms warfare from Imperial Army instructors and Lunar Templars sent to supervise the training of the first batch of Auxiliaries by Horus. 

As such, Cthonian Regiments follow standard Imperial Army doctrine as taught by the Tactica Imperialis with little local variation save for a general rebellious attitude towards protocol and tradition as well as a greater emphasis on small unit tactics due to the aggressive gung-ho spirit of your average Cthonian.

This has caused a strange paradox of Cthonian regiments oddly easily integrating within larger Imperial Army formations, while also at the same time earning them a reputation of being hard to work with and unruly. 

Cthonians regiments can perform almost any task demanded of them from desperate holding actions to rapid maneuver warfare. One area in which they particularly excel is CQB combat, urban warfare, and tunnel fighting. This is not due to any particular difference in their training, equipment, or organizational structure, but because of the cavernous nature of their homeworld. 

Organization

The Imperium generally allows regiments to organize themselves in any way they see fit so long as they remain combat-effective and meet the minimum personnel requirement of 5,000 combat troops.

Again though, due to the complete lack of any cohesive organizational structure on Cthonia before Compliance, Cthonian regiments follow the organizational templates laid down in the Tactica Imperialis. 

Cthonian Regiments possess 8,000 men under arms, led by a Colonel, and are organized in a square structure. The regiment is divided into four brigades, led by majors, each with 2,000 men and women. These are split into four battalions, led by commanders, of roughly equal size which are further split into four companies, led by captains. Companies are split into three platoons, led by first or second lieutenants, which are finally split into 4 squads led by sergeants.