Duty's Mark

Where many chapters identify their Sergeants through red helmets or similarly stand out colours, the story behind the Iron Seraphs iconography is quite different. The 'Duty's Mark' is the unofficial name of the practice within the Iron Seraphs to denote Sergeants using a deep red on their right arms. Part of the reason for this is to symbolize the individual's experience with their armour having its colour overtaken with evidence of their long service to the chapter. But the main reason lies in the story of one of the many heroes of the Chapter. 
During one of the many conflicts in the Chapter's history, a small element of the 5th Company was facing off against raiding armies of the Drukhari of size and organization unseen in that region of Imperial Space. While the Seraph's defensive lines stood impenetrable, the collateral damage and use of warp magics to burn the settlements around them were prolific, often undermining the defensive nature of the chapter. Attempts at attack were often blunted as the raiding parties would quickly retreat once the Seraphs had advanced far enough out of position. 
With few marines and limited support, the Chapter was forced to continue these brief and intense battles, often trading very favorably against the Drukhari but losing the targets they were supposed to be defending in the process. 
Things would come to a head in a final battle of unexpected ferocity where the Seraphs seemed to be set upon by all the available Drukhari forces. Hundreds of them descended on the tens of Seraphs with many being torn down by the rattling bolterfire of the defensive lines. Yet still the Drukhari seemed hellbent on breaking these lines, goring themselves against the armor and weapons of battle brothers for the slightest chance to slay command personnel. The astartes were eventually forced into a compact defensive circle to cover their flanks while the Drukhari stormed around them, trying and failing to break through as plumes of dust, smoke, and debris nearly blotted out the sun. At this moment, Battle-Brother Etanos, witnessing the apparent death of Brother-Sergeant Hencolus, surged into action.
His right arm glistened red, covered in blood and gore from himself, his battle-brothers, and the countless enemies he had slain. He raised it high above their battle lines, through the choking smog and dust of the Drukhari and catching what few beams of sunlight could penetrate through, he called out to his brothers, rallying them to his side before leading a near unstoppable breakout directly towards the Archon leading the Drukhari forces.
The Seraphs surged with fervor, following the shine of the red fist through the smoke and dust as they barreled through the Drukhari and tore through their forces with blade and fist before finally reaching the unsuspecting Archon. In his final moments, he seemed to call out to retreat, to try and escape, watching as his elite guards seemed to collapse against the seemingly deathless and undaunted charging wall of ceramite and rage. But Brother Etanos would seize his soul for himself
"YOU DARE INSULT US WITH RETREAT!?"
He tore the arms off the Archon,
"YOU SPILL MY BROTHER'S BLOOD AND INSULT THE DUTY OF MY CHAPTER!?"
A leg this time, stomped into dust and mangled flesh under the Etanos' boot,
"CARRY THIS MESSAGE BACK TO THE REST OF YOUR DOOMED RACE: WE ARE THE UNBROKEN ANGELS, WE ARE THE IMMUTABLE SHIELD OF THE IMPERIUM, WE ARE SERAPHS FORGED FROM IRON AND WITH YOUR DEATH WE PAY HONOUR TO THE GREAT DREAM OF THE EMPEROR!"
A single furious strike to the Archon's chest splatters him across the battlefield.
Brother Etanos' words seemed to reverberate through the battlefield, riling the Seraphs into a frenzy at his honoured words, all in the name of their duty. They went seemingly unaffected by gouged armour, countless lacerations and whips tearing into their armor, any attempts by the Drukhari to try and escape or reform some semblance of organization were dashed as the remaining marines seemed to chase them down, their retreats actively enraging the astartes as if a blight on their very name. 
What few managed to escape into the Webway did so covered in gore and whimpering in fear as they barely managed to escape from the seemingly crazed astartes. 
Back on the world, the few marines which remained numbered less than fifteen compared to their original thirty. With the retreat of the Drukhari, the Seraphs tended to their dead and wounded, recovering bodies and carrying their Battle Brothers to the nearest settlement. Brother Etanos had been wounded beyond healing but managed to stay conscious long enough for a wounded Brother-Sergeant Hencolus to meet him. His deeds would be remembered not simply through the history of the chapter, but his gleaming red fist would become the symbol of leaders who braved the horrors of battle but stood undaunted, the symbol of Sergeants and the duty they were charged with.