Mordant Acid Dogs
The Mordant Acid Dogs are a regiment of Imperial Guardsmen, and some of the ballsiest in the entire Imperium, so brave and insane that even a Krieger would nod in suicidal respect to them.
The entire story behind them is they come from a planet that has a bacteria that helps produce industrial acids. The planet itself is barely habitable, and almost all society and work takes place underground and in caverns. The mining for this, as is to be expected in the grimdark of the 41st millenium, is controlled by gangs. Officials on Mordant are happy to let them squabble, so long as the quotas are met.
This results in a "survival of the fittest" society, where the weak die and the strong survive. Unsurprisingly, there is also a rampant, excessive, borderline zealous hatred for mutants on Mordant, which seem to be quite common for a mysterious reason. Despite this, and typically being jaded individuals, they are nonetheless idealistic individuals who are fierce ass soldiers and excellent underground, tunneling, and hazardous environment combatants. As you would expect, they have adapted to low light conditions quite well, and consider normal light to be painful, often wearing specialized glasses in "bright" environments.
Let's Hear Some Lore
A group of Acid dogs were deployed to these mutant infested planets called the Gloom worlds. Eventually Colonel Bane, the commanding officer of the 13th Mordant, realized that these mutants were too organized to be operating on their own.
Said suspicions were proven correct when a bunch of Slaaneshi cultists dropped from the sky and slaughtered the garrison Cadians. Reluctantly, Colonel Bane ordered his troops to bombard the Cadians in order to thin the enemy numbers. He then ordered his men to march upon the landing site, unsure of what they may find.
The ensuing antics included Bane and his men charging the Chaos lord. Mordants slaughtered demons, Noise Marines, mutants, and cultists left and right on a scale that would make most Space Marines quake. What makes them even more awesome is that most of them were possibly tainted by Chaos during a previous engagement. They resisted the lure of chaos through pure determination.
Truly, they were heroes of the Imperium like no other. The Inquisition themselves stepped in to ensure that these brave heroes were suitably rewarded; specifically, by an orbital bombardment.
Presumably, this was done out of pure jealousy but the Inquisition cobbled together some bullshit story about chaotic taint for the gullible.
- The video that does them more justice than this article ever could. https://www.youtube.com/watch?v=lYh6GzVsXIM&t=192s