Editing Hua Yuan Exterminators (section)

===History and Background===
The concept of the Exterminator combat doctrine is evolution in its purest form. Before the Praetorians discovered the lonely moon, the gangs and sects ruled the hive, whilst the proto-nobility manipulated these forces against each other to maintain their power. The Exterminator practice of free-running was born in this urban crucible, sliding around the sloped sides of the burgeoning hive, and sprinting in and out of the metal labyrinth the quickest and easiest way around the wreck of the Brimstone. Once the hive blossomed in full the art of freerunning, known as Yùnxíng miǎnfèi or simply Yunyun in the hive, expanded. Some gangs are legendary for their acrobatic ability, especially in the upper levels of the mid-hive, where one can better jump off roofs and actually get some air.  The current urban fighting doctrine is also a formalized fighting style developed in the hive, where the battlefield is is not a simple plane, but a multitiered chessboard, in which the flow of battle is in constant motion. A Hua Yuanese fighter must be constantly aware of what's around them, even from above and below, hyperfocused on their surroundings. This coupled with agile troops makes for a highly reactive fighting force, never one to hold ground for long. 

The most famous aspect of Exterminator Warfare, dubbed "Color and Mayhem", was actually a product of the enforcer gangs and PDF created by the nobility. Typically gang warfare was naturally quick and violent, but without much in the way of pretense. Using concussion grenades and tear gas was the work of law enforcement. Gangers typically liked to ambush their quarry using vehicle borne improvised explosives and simple rushing assaults. The transient and quick paced flow of battle was combined with the raid style opening of combat, which ultimately co-evolved into making the battle space as usable as possible, in the Tyranid invasion.

The Genestealer cult was quietly flourishing in the bowels of the ship, then expanded even further when the hive began to grow around the wreck. By the time the PDF and gangs where finding a common enemy in something that wasn't entirely a gang or a cult, the splinter had already found them. When the hive ships, tossed into orbit by the abating warp storms and subsequently crashing, arrived in Hua Yuan. everyone that could hold a weapon became a soldier, and the line between sect, gang, and enforcer was eradicated. The unlivable atmosphere, coupled with the absolutely barren landscape forced almost all of the invasion to take place within the hive. It was here, in the event known as the Fall of Seven Walls, in which the hive was beset on all sides. The gangs, cults heretic and faithful alike, and those who upheld the law banded together to repel the Devourer. Surgical precision met chaotic and frantic fighting. The gangs taught the law the delicate art of fighting dirty, and the PDF instructed the criminal element on how to properly conduct themselves in the field of battle, and the sects provided a sense of fearlessness and ruthless fanaticism that only those who believe their souls saved could.

The Tyranid threat forced them into a cohesive fighting force, and in a surprising show of solidarity, all levels of the hive where fighting as one. The Hive Mind saw that mass formations of swarming hives was proving to be ineffective, as the humans were willing to incinerate entire Hab-Blocks to deny the swarm biomass. Forced to assimilate whole cells of the Genestealer cult in order to keep numbers up, the Hive Mind adjusted its tactics rather quickly, shifting resources from ranged bioforms to more specialized bioforms such as Lictor and Genestealer broods capable of assassinating whole command structures and carnifex broods that were capable of dismantling entire hab blocks by themselves. This change in warfare caught the Hua Yuanese off guard, and the defensive was slowly becoming dismantled from within by genestealer hybrid infiltrators and assassin bioforms.

One invention finally turned the tide of war. The genesis of this dubiously heretical Combat Stim is shrouded in mystery, but its use was the catalyst that drove the Tyranid threat into near extinction. Named Xenon after the pale glow it exhibits when it it is first made similar to the element of the same name, Tyranid Dust was used to root out Hybrid's in their ranks. Although higher generations of hybrid could escape the notice of individuals under the effects of Xenon, the use of the drug saved untold lives and prevented a plethora of assassinations that would have spelled the end of the fledgling hive. 

With Xenon in the arsenal of the most dedicated fighters, they developed the less famous style of fighting that gave them their name. Exterminatus.

The fighters realized, like so many before them, that the only way to defeat the enemy before them was by being relentless in their removal. They had but one choice; to extinguish the Tyranid threat completely, no matter the cost, no matter how. To this end the element Kaihonium was used as a cheap alternative to Holy Promethium, entire floors set ablaze with green, orange, blue, and pink fluorescent flame.

Others sealed off hab-blocks systematically and flooded them with poisons. Every time they would concocted a new breed of toxic fume, and they would unleash it upon the enemy, rarely allowing the Hive Mind to adjust to their tools of extermination. Hunting them through the hive these exterminators razed the city. When they could pinpoint a brood they would descend upon their quarry like hawks, busting into a hab-block and clearing it out of any and all life. If they could not find the source of the infestation they would simply ignite the area, or conduct horrifying chemical and biological warfare. It was an effective strategy, and soon the brood found itself with too little biomass to continue the offensive. The Tyranids fled underground, deep within the world's crust. 

The Exterminators helped rebuild and rooted out what they believed to be the last of the Tyranids. When the Praetorians finally arrived they were inducted into the Imperial Guard wholesale, the enigmatic enforcer captain Yeoh Hong forged into a commanding officer for the regiment, and one of the most famous figures in the division.