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.
Ok, but why are they so great?
To answer that retarded question, lets go on to the following story.
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 perverts dropped from the sky and slaughtered a bunch of Cadians. Reluctantly, 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 resulting awesome that resulted included Bane charging with his men upon the Chaos lord and his contingent. What many believe is that the Mordants literally HATED the heretics so much that they gained the Emprah's protection and strength, and proceeded to slaughter demons, Noise Marines, mutants, and cultists left and right. What makes them even more awesome is that most of them were possibly tainted by Chaos during a previous engagement. Nonetheless, they decided they weren't pussies, and through sheer determination, resisted the lure of chaos and cleansed the world they were on of organized Chaotic taint. Truly, they were heroes of the Imperium like no other. In fact, they made the Inquisition so jealous that they decided to nuke them from orbit. Ok, the real reason was the Chaotic taint, but we all know the truth, don't we?
- The video that does them more justice than this article ever could. https://www.youtube.com/watch?v=lYh6GzVsXIM&t=192s