Drop Site Massacre

Probably how it looked like without thousands of dead Astartes and tanks lying around.

"...Stories told that, in a time consigned to legend, Father Isstvan himself had sung the wold into being with music for his Warsingers to hear and interpret. Father Isstvan was, it seemed, a fertile god and had spread his seed far and wide across the stars, nameless mothers bearing him countless children with which he had populated the first ages of the world. ...the myths of Isstvan, which told of the children of Father Isstvan who turned from his light and led their hosts against their benevolent sire. A terrible war followed. The Lost Children, as they came to be known, were finally defeated in a great battle and their armies destroyed. Instead of slaying his wayward children, Father Isstvan banished them to Isstvan V, a desolate place of black deserts and ashen wastelands."''

"The Battle of Isstvan V", "That Huge Fuckup", or "The Drop Site Massacre" as it would be better remembered as later, was one of the major defining battles of the Horus Heresy, that occurred circa 4 months after the Battle of Isstvan III. After the revelation that Horus had flipped him the bird, the Emperor threw a huge tantrum and sent the loyal primarchs and their Legions to deal with the traitors. Ferrus Manus, Vulkan, and Corax, being the closest, were sent as the vanguard of the major Imperial force. The vanguard legions would be the hammer that would break the traitors against the anvil that would be the Iron Warriors, Night Lords, Word Bearers, and Alpha Legion.

Little did the loyalists know that the anvil has already betrayed the Emperor before arriving at Isstvan V. By this time, fully half of all primarchs and their respective Legions have been swayed by Horus in either joining him in his path to becoming the rightful leader of Mankind or by realizing that the Chaos Gods are much more favorable subjects of worship.

And these legions would now have to fight it out like adult men pumped by the barrels of butt-hurt, big guns and testosterone on the depressingly lifeless deserts of Isstvan V.

The Battle

Iron Hands at the Drop Site Massacre, shortly before their Primarch loses his head.

While making battle with the remnants of the loyalists on Isstvan III, Horus sent Fulgrim to The Urgall Depression, a volcanic valley on Isstvan V, to dig trenches, build barricades and bunkers and to fortify the ancient abandoned fortress complexes there. Imagine the fabulous, pinky Emperor's Children, staining their perfectly blonde hair and digging in the mud. Yeah, the decision did not sit well with Fulgrim, but he did his job well. Fulgrim and his men built a ton of anti-air and -missile batteries, forcing the attackers to refrain from Exterminatus everyone from orbit and being done with it.

This was Astartes business, so the Mechanicum and the Guard were not present on either side, except for some Legio Cybernetica robots of the Word Bearers. The battle went very well for the loyalists, who had the advantage of numbers and all that pain and raeg to vent. Mortarion, Angron and Fulgrim led their respective Legions on the ground, while Horus probably sat on a balcony in the fortress like a total boss, orchestrating the whole traitor army by whispering commands to everyone through the vox. In addition, Corax, Ferrus Manus and Vulcan were present from the loyal side of the primarchs while Lorgar and Konrad Kurze were also present.

The Iron Hands, Salamanders, and Raven Guard led the first and only assault on the traitor fortress complex, while awaiting reinforcements from the other 4 legions. When those arrived, the Salamanders and Raven Guard fell back to catch a breath and resupply, while the Iron Hands decided to push forwards over the retreating pretending-to-be-panicking traitors. As the loyalists neared their initial LZ, the newly arrived loyalists opened fire, revealing their true allegiance. As this happened, the Sons of Horus, World Eaters, Emperor's Children, and Death Guardattacked from the rear, completely encircling them. The loyalists were caught completely off guard and were fucked over almost as hard as these guys. Perhaps even worse was the loss of two of the loyal primarchs, as Ferrus Manus received a Classic Slaaneshi Daemonette Treatment (meaning the subject's head is chopped off exactly the same second the orgasm is reached, resulting in a forever erect and stone-hard penis), and Vulkan disappeared, before turning up again pretty fucking quickly after pissing off papa emo himself, which resulted in him becoming insane just long enough to get dicked over again.

The loyalists were forced to perform an emergency extraction and barely managed to escape, however the three legions were crippled and largely unable to participate in the Heresy after the massacre (no shit). The Massacre remains a bitter memory for the three loyalist legions and their successor chapters ten-thousand years later. Each legion lost between 75 and 85 % of their numbers in just a few hours. The Raven Guard would get even more thoroughly fucked over, trying to recover their numbers. The Salamanders, being already one of the smallest legions, would not even have enough marines to fill their own chapter, let alone a successor at the 2nd founding. The Iron Hands and their primarch would forever become the butt of beheading jokes.

Iron Hands Perspective

Being extremely pissed of at Fulgrim, Ferrus Manus could not hold his load and decided to lead the assault against the traitors being holed up in their fortified fortress. Being the tough bastards they are, they set the standard for the future generations of Iron Hands and their successors to come, during this battle.

Emperor's Children Perspective

"Horus has seen the truth of things, my brother. The Emperor has already abandoned us and even now plots hos apotheosis. Helied to us all, Ferrus. We were nothing more than tools to win back the galaxy in preparations hos his ascension! The perfect being he pretended to be was a filthy lie!" - Fulgrim's first attempt to sway Ferrus Manus to Chaos.

Fulgrim was massively disappointed and depressed that Ferrus Manus would not be swayed to Horus's side and that he would be forced to die (Horus's own words). By failing to convert his ugly bro in his own flagship, Fulgrim knocked Ferrus out with a hammer-blow to the jaw that would send an Astartes head flying for a mile, and fled into the warp to deliver the bad news to Horus.

Shortly thereafter, the transformations of the Emperor's Children into the metal- and shoegaze-loving Noise Marines that we know and love, has been complete, both chaotically and genetically. Fabulous Bile made sure of adding a ton of physical and genetic alterations to his brothers, like speed enhancements and having the ability to produce ear-chattering, bone-breaking sonic waves by opening the mouth a little to wide, flailing ones arms in circles and generally looking like a skinned bear recently castrated without anesthesia.