Drop Site Massacre
"...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."
"This betrayal would stain forever the honor of the Astartes, no matter the outcome. Men will fear us from this day onward, and they will be right to"
- – Ferrus Manus
"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: the Sons of Horus, World Eaters, the Emperor's Children and Death Guard. The Iron Hands, Salamanders, and Raven Guard 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
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 or his men, but they did their job well in spite of their insistence that the Iron Warriors should've done it instead. 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. The loyalists bombarded the site from orbit anyway, but to little effect. Of course they could have just kept shooting as that is a form of exterminatus and eventually the traitors would have been wiped out. Like Horus did at Istvaan III after realizing the surviving loylists were winning.
Nearly 30.000 Astartes (probably closer to 300k thanks to retcon), the Mechanicum's Dies Irae (the Warlord IMPERATOR Titan present at Isstvan III), other Dark Mechanicum machines and the Guard artillery pieces were on the defense. Nearly 40.000 loyalists (again, more like 350k) were on the offense and went about, venting their raeg after deploying á la STEHL RÄHN tactics into the middle of the defenders. (Baneblades?) Felblades and its variants, Land Raiders, Predators, Whirlwind, and Army artillery deployed behind, on the hills and around the depression.
Mortarion, Fulgrim and Angron actively 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, to the loyal Corax, Ferrus Manus and Vulkan, four additional traitor primarchs arrived later: Lorgar, Alpharius, Perturabo and Konrad Curze.
Imagine around 70.000 Astartes (More like over a MILLION Space Marines!) + all the vehicles of their legions + puny humans and their artillery in a terribly confined space, in a line less than 20 kilometers wide. Armies that could conquer entire planetary systems were crammed into a medium sized, open valley. The blood ran in rivers, the slaughter magnificent.
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 (fresh and more than 40.000 of them) opened fire, revealing their true allegiance. As this happened, the Sons of Horus, World Eaters, Emperor's Children, and Death Guard attacked 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 face of most beheading jokes.
Emperor's Children Perspective
"Horus has seen the truth of things, my brother. The Emperor has already abandoned us and even now plots his apotheosis. He lied to us all, Ferrus. We were nothing more than tools to win back the galaxy in preparations 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-shattering, bone-breaking sonic waves by opening the mouth a little too wide, flailing ones arms in circles and generally looking like a skinned bear recently castrated without anesthesia.
In the center of the traitor line, the Emperor's Children fought with unremitting cruelty, it's warriors howling with savage glee as they killed their former brothers. Unnatural horrors of mutilations and degradation were visited upon the living and the dead as Fulgrim's Legion repulsed every attack. Bizarrely clad warriors in Mark IV plate draped in stretched skin cavorted in the midst of the deadliest combats, fighting without helmets, their jaws wired open as they unleashed a hideous screaming. They bore unknown weaponry and fired echoing blasts of atonal harmonics that ripped bloody canyons in the massed ranks of the Iron Hands. Great pipes and loudspeakers fixed to their armour amplified the screaming vibrations of their killing music, and deafening sound waves tore apart warriors and armored vehicles.
Lastly, in the ending moments of the battle, Fulgrim chose to reveal himself and meet the Gorgon's challenge. The duel that took place next, was incredible in it's emotional proportions and significance, so go and read the fifth HH book, "Fulgrim". It's an awesome read. Spoiler: Notably Fulgrim attempted o resist the Laer Sword which had ultimately corrupted him one last time by pulling the mortal blow on his bro Ferrus, regaining a last moment of clarity when seeing what had become of his legion. The Greater daemon in the Sword took no chances, forced Fulgrim into killing his bff Ferrus and then gave him the oblivion he craved following the murder (by possessing his body, d'oh)
Word Bearers Perspective
By the time the legion of the first heretic arrived at Isstvan, the first possessed Space Marines, the so called Gal Vorbak ("Blessed Sons") were now a part of the Word Bearers, as well as apostates (their chaplains) with the power to summon daemons. The story of the first possessed marines is incredibly interesting and can be fully enjoyed by reading the HH novel "The First Heretic". The book also details the confrontation between Corax, Lorgar and then Curze; really cool shit. The first mention of the Legio Cybernetica robots is also here and present during the battle.
Lorgar showed hesitation all this time and even during the battle itself. He felt shame for causing so much chaos and bloodshed, when he only wanted to topple Big E from his throne. He had never imagined the scale of this betrayal. And thus, when he saw Corax slaughtering the possessed marines, he ran towards Corax with no hope of surviving the confrontation and not caring either in equal measure - against his most "trustworthy" advisors Erebus and Kor Phaeron (who almost got a heart attack when Lorgar decided to engage) -. Such a sensitive soul, don't you think? *sob*.
"- I will kill you, Lorgar."
"- I know. But I have seen what will be. Our father, a bloodless corpse enthroned upon gold, and screaming into the void forever." But before Corax could finish the job Lorgar was conveniently saved by the greatest psychopath under Horus' command (i.e. Kurze)... well maybe Angron with his fried brain might be another contender for "most crazy up primarch" (with his tourette syndrome twitches and anger tantrums and all), but that's another story. And no, Fulgrim doesn't count. He was too busy getting possessed (and to his credit if the EMPRA had not lied to all of his sons the whole heresy might not even have begun in the first place as Fulgrim had no chance of fending off his corruption without even knowing of CHAOS and where the strange voices in his head came from... Once he finally realized that the insidious whispers were actually real he was already too far down on the road to damnation for it to matter. I mean he was looking at a picture made of excrements and other bodily fluids by the time and mind you that was before he officially turned traitor)
Iron Hands Perspective
Being extremely pissed of at Fulgrim, The Gorgon could not hold his load and decided to lead the assault against the traitors holed up in their fortress. Proving once again that they are tough bastards in a fight, they set the standard for the future generations of Iron Hands and their successor chapters, during this battle. Even though they nearly got wiped out, they recovered eventually enough to give birth to successor chapters like Red Talons.
Two late arrivals, Shadrak Meduson and Autek Mor, gathered as many survivors as they could - Meduson piecing together a coalition of Iron Hands, Ravens and Salamanders - and embarked on two epic, but very different guerilla campaigns against Horus. Letˋs not forget that the cybernetic robot marines of the 40k still cling to their delusion, errr I mean entirely logical conviction, that Ferrus would have lived if the Salamanders and Raven Guard just had had as much balls of steel as they had and died a manly death in the crossfire of the traitors instead of trying to disengage, cowardly bastards. Another proof that the flesh is weak. Wait, what organic part can I replace next?
Raven Guard Perspective
The screaming raptors of the Raven Guard cut a swathe through the enemy's right flank, his fearsome assault wings dropping from above on the fire of jump packs, and slaughtering their foes with shrieking sweeps of curled blades.
While the other legions lost the biggest percentage of their numbers, the Raven Guard lost the most in pure numbers, which led Corax to resort to technology from the Dark Age to replenish his ranks. That can be read in HH novel "Deliverance Lost".
The ground murmured with his landing. Claws slashed from their power-fist housing with silver flashes, and shimmering wings of dark metal reached up from his shoulders into the air above. Slowly, so painfully slowly, he raised his head to the traitors. Black eyes stared from a face whiter than Imperial Marble, and written across the pale features was the most consummate, complete anger Argel Tal had ever seen. It was an emotion truer and deeper even that the rage that ruined the faces of the daemons within the warp. But it was not anger, nor rage. It went beyond both. This was wrath, in physical form.
Salamander's Perspective
"...the mighty figure of Vulkan strode through the torrents of bolts, killing with every sweep of his sword and shot of the weapon his brother Ferrus had forged in his name. A colossal explosion erupted at the primarch's feet, wreathing him in killing fire, and dozens of his Firedrakes were hurled through the air, their armour molten and the flesh seared from their bones. Vulkan marched through the fire unscathed, continuing to kill traitors without missing a beat."
- – from the eyes of Gabriel Santor, First Company Captain of the Iron Hands.
The End Note
All of the traitor legions and primarchs were present at Isstvan V (except Magnus the Red), with almost all their forces as well. Five of them would strike directly at Terra, especially now that Mars was under Dark Mechanicum control. The Alpha Legion was sent to harass the White Scars who were on their way to Prospero to figure out what the hell had happened to the Thousand Sons, since the Khan had been friends with Magnus and wanted to know if the rumours about the Wolves fucking the place up were true. At that point he did not even really know what exactly was going on, only that shit had been hitting the fan. Night Lords were sent to some important system in the Eastern Fringe, to terrorize and eventually take over some still-loyal Forge Worlds there. Perturabo was permitted to send his fleet to vent his specially reserved frustration on an Imperial Fist armada that had not made it to Isstvan V in time and was mustering at Phall where he would have had his ass handed to him by a lowly Space Marine Captain (so much for the vastly superior intellect of the Primarchs) if they had not been forced to disengage. To make things worse, if the Imperial Fists fleet had not received the order from Terra at the exact moment the assault on Perturabo's flagship chances are they might even have been successful (as that way the planned 2nd wave never manifested and the Imperial Fists on the Iron Warriors flagship were slaughtered alongside most of the armada being massacred as they tried to disengage). The short story ends with Perturabo running the calculations and realizing he would have lost the battle.