Battle of Isstvan III

Isstvan III
Date	005.M31
Scale	Planetary
Theatre	Horus Heresy
Status	Traitor Victory
Belligerents
Traitor Legions	Loyalist Elements
Commanders and Leaders
Horus, Angron, Mortarion, Lord Commander Eidolon	Saul Tarvitz, Garviel Loken and others.
Strength
Sons of Horus, Death Guard, World Eaters, Emperor's Children Legions, plus support from Titan Legions.	~100,000 Loyalist Marines drawn from Traitor Legions, plus support from Adeptus Mechanicum loyalists. Of those, approximately 60,000 would survive the initial bombardment.
Losses
Approximately 40,000 marines.	Nearly all forces, except for Garviel Loken, Crysos Morturg and Calleb Decima. (Special Note: Only around 20,000 loyalists died during the actual fighting, the rest likely were slain in the final bombardment)(meaning the loyalists kicked the shit out of the traitors despite being murderized by Primarchs and Titans)
Outcome
Loyalists nigh-annihilated. The outcome was never in any doubt, though the stubborn defense by the loyalists cost the traitors more in time and manpower than they had anticipated.

The destruction that took place in the battle was fucking GLORIOUS!!! The planet would have been literally on fire the whole time, too, thanks to the virus bombing and the following fire wave.

"Order the guns to fire! Let the galaxy burn!"

– Warmaster Horus Lupercal, unleashing the Virus bombs on Isstvan III

"This is treachery! All of this! We are betrayed. The fleet is going to bombard the planet's surface with virus bombs."
"What?" spluttered Garro, disbelief plain in his voice, "That's insane."
"Trust me," said Tarvitz, "I know how it sounds, but as my honour brother I ask you to trust me like you have never trusted me before. On my life I swear I do not lie to you, Nathaniel."
"I don't know, Saul," said Garro."
"Nathaniel!" screamed Tarvitz in frustration. "Ship to surface vox has been shut off, so unless I can get a warning down there, every Astartes on Isstvan III is going to die!"

– Saul Tarvitz & Nathaniel Garro learning of the upcoming betrayal

The Battle of Isstvan III was the first official battle of the Horus Heresy, carried out by the Sons of Horus, the Death Guard, the Emperor's Children and the World Eaters, to purge any remaining loyalists from their ranks. Mathematically, the combined traitor legions were down 140,000 marines by the end of the battle as well as all the Titans and other Mechanicum assets they lost in the fighting. As such, they really wouldn't have had a chance at victory in the rest of the rebellion if not for the later Drop-Site Massacre.

As the final step before declaring war on his father, Horus needed to get his brothers on his side and purge their legions of those who could not be swayed to betray the Emperor or whose loyalties were unclear. Horus wanted only soldiers utterly committed to his cause at his side. He used the compliance campaign of the Isstvan system as a cover up for this activity. Everyone that died at Isstvan III, would be said to have died to a biological weapon deployed by the natives. He, Angron, Mortarion and Lord Commander Eidolon (Horus had sent Fulgrim to try and sway Ferrus Manus to their cause), hand-picked the squads that would form the first wave of the invasion, rather than committing entire companies as was the standard operating procedure.

Once the initial wave landed, the Primarchs set about deploying virus bombs to the fleet. This was where things didn't go according to plan. Saul Tarvitz, Captain of the Emperor's Children 10th Company was the type of fellow that questioned everything around him, the type of guy that had a pretty logical mind, compared to most other Astartes. When he considered the fact that Eidolon, a notorious glory hog, would not be leading the speartip against the Isstvanians and the fact that all the attackers were made up of random, hand-picked squads that had nothing to do with each other normally during a deployment, he became paranoid and sounded his way into staying aboard the space ship Andronius. Saul smelled fish and he would find out why it smelled of fish on the ship. And what he found out was that the ship was arming the soon-to-deploy virus bombs. Not only that, but it was also moving into lower tiers of atmosphere, the default position for bombardment from space. This was in the middle of the battle down on the planet, where Saul knew his battle brothers were still fighting. So he acted.

So he stole a Thunderhawk and took off to warn his brothers on the ground, because he didn't trust the vox communications. On his way there, he warned his closest bro, Nathaniel Garro aboard the Eisenstein of the Warmaster's plan. Upon discovering that virus bombs were being prepared to fire from their ship, Garro's company took control of the ship, killing the traitors under the command of Ignatius Grulgor who had begun to prepare to turn on the Loyalist crew so they could join the bombardment themselves, and escaped the system after accepting refugees Iacton Qruze of the Luna Wolves, remembrancers Euphrati Keeler (the first Saint), Mersadie Oliton (who recorded everything in her specifically modified brain), and the prime iterator Kyril Sindermann (later a founding member of the Inquisition), who had witnessed Horus's massacre of the remembrancers and civilians on his flag-ship, The Vengeful Spirit. For no apparent reason they did not use the shuttles and other vessels such a large ship would have to swiftly ferry the loyalists on-board before the bombardment. Maybe because then the traitors would have been crushed by the Imperial response and we'd have no grimdark.

Saul first warned Lucius and the Emperor's Children, who then relayed the message to the rest of the ground forces. Not everyone could be saved from the virus bombing and the usual bombardment with fire afterwards, but those who did not hide in the bunkers and catacombs were exterminated like the rest of all life on Isstvan III. When the loyalists emerged, all life, down to even single-celled organisms, were purged from the planet. Even so, it says a lot about their armor and enhanced bodies that they survived with such methods. Anyone or anything trying the same method would have easily died anyway from the convection.

Battle for the Precentor's Palace or the Three-Primarch Rumble-Derp

"We can really make them suffer, Garvi, we can force them into a battle and delay them. If Tarvitz got the warning out here, who's to say that there aren't others carrying a warning to Terra? Maybe the other Legions already know what happened. Someone underestimates us, they thought this would be a massacre, but we'll go one better. We'll turn Isstvan 3 into a war."
"Do you think we can?" said Loken.
"We are Luna Wolves, Garvi. We can do anything."

– Tarik Torgaddon, giving the peppiest of peppy pep-talks to Garviel Loken, before they linked up with the survivors from the initial bombardment.

When Saul Tarvitz was on the ground, he took command of all remaining loyalists alongside Garviel Loken and Tarik Torgaddon, the Captains of the Luna Wolves' 10th and 2nd Companies. Regardless of their original legions and companies, the men looked up to him as their natural, humble leader, who, by showing insight and independent thinking saved them once before, could save them again. And he did. Time and time again, against the offensives launched by the traitors. They held out against the traitors for 3 FUCKING MONTHS. 2000 Astartes with no supply lines or tanks. Not even METAL BOXES. Against 4 legions, Mortarion, and Angron.

Speaking of that puppet, we probably have him to thank for this, because before Horus could bombard the loyalists into ash, he went down to the surface himself (various speculations are that he was goaded into it by the insults of the surviving loyalists, that he was too impatient to wait for a second bombardment, that he was disgusted by the idea of his sons being killed by a virus bomb and decided to give his sons an honorable death instead 'cause it reminds him too much of his fellow slaves, or that he was just being an idiot that wanted to RIP AND TEAR). As bombarding the planet a second time with Angron on it was not an option (lest the World Eaters turn on Horus for bombing their primarch and making an enemy of Angron if he survived), Horus had to deploy the rest of his forces alongside Angron, including Mortarion and his own Captains Ezekyle Abbadon and "Little Horus" Aximund, resulting in an inefficient, time consuming and bloody battle that pushed back his timetable even though it hardened the resolve of the traitor forces against the loyalists and blooded them to the point of no return.

"Emperor's Children do not run!"
"They do from this!"

– Captain Erhlen of the World Eaters, talking Saul Tarvitz out of a double date with Gorefather and Gorechild.

But really, the fact that the Loyalists survived so long speaks of some serious level of heroic badassery from Loken and Tarvitz leading the loyal forces and some serious derp on the part of the traitors. Even Mortarion himself spent three months fighting his own legion in the trenches on the northern part of the Palace. Simply put, the amount of some serious shenanigans the Loyalists had to pull off to survive for so long... it's legendary. Maybe, the Emperor was already protecting them to whittle down the traitors into a manageable force.

The Death Guard were kept busy besieging their own Loyalists, the Destroyer captain Crysos Moturg taking command with the Grand Company Captain who'd led them before having died in the bombardment.

One can only guess for how long the loyalists would have held out against the traitors if Lucius had not betrayed them. That double-crossing bastard was the most egoistical motherfucker in the whole 40K universe. He killed his own squad-mates and let Eidolon's Emperor's Children Terminators breach the defenses unchallenged, all because he was jealous that Tarvitz was chosen to be the leader of the loyalist Emperor's Children instead of him. It is quite odd that he did, though. There were three months of opportunities he had yet did not and he clearly hated the traitors otherwise he wouldn't be fighting for the loyalists. Not to mention wouldn't it be far more infuriating that your own brothers and father are trying to kill you? He definitely is not stupid, either, so dealing with the people actively trying to murder him should have come before personal issues. He could always try to murder Tarvitz when doing so wouldn't risk getting himself killed by enemy combatants. But then we wouldn't have Warhammer 40,000: Grimdark, we'd have Warhammer 30,000: Human Utopia instead. On the other hand he had already unknowingly been corrupted by a remembrancer (see the novel "Fulgrim" for details) who convinced him to scar his face as Loken had destroyed his beautiful face already anyway. So, one might as well argue that it simply took 3 months for the corruption to fully take root, until he was finally ready to betray his best friend Saul.

Even that swell guy got his ass handed to him by Loken and later impaled on a Land Raider's dozer blade. He survived though, because he is the Eightfold Path. Even Lucius got his ass handed to him by Tarvitz and later chased away like a dog with bolter fire from fellow loyalists. Even the attack launched by Eidolon, thanks to Lucius, was repelled and cut down by a counter-counter attack led by Tarvitz after his duel with Lucius. Even Abaddon was forced to pull back after a duel with his former battle-brother Loken, although it left Loken badly wounded to the point where everyone thought he had died.

You'd really have to read the book "Galaxy in Flames" to experience the full detail of this battle. The last part of the book, "Brothers" is just a heart wrenching read that would put a ground-standard for all bad assery that would come after those that were shown by the loyalists during this battle.

It's a shame that it all ended in fire, Saul Tarvitz and his remaining 100 loyalists were blasted from orbit, instead of dying in a final assault. But this does underscore how awesome they were. Horus didn't want to risk getting bogged down again by just those one hundred Marines and didn't want to take the significant casualties he (rightly) expected in further combat with them.

The flight of the Eisenstein allowed Nathaniel Garro to warn the Imperium of Horus's treachery when they made contact with Rogal Dorn. The ground battle cost Horus time that he needed to consolidate his gains and steam roll towards Terra. Rogal Dorn organized a fleet of seven eight legions that would eventually reach the Isstvan system and challenge Horus in his fortified position on the Urgall Depression of Isstvan V.

Nothing could possibly go wrong.

Tabletop

Forge World's first Horus Heresy rulebook, Betrayal, is focused on this battle. In addition to the generic rules for Space Marine Legions and a Mechanicum detachment, it includes special rules for the specific Legions and characters involved, including Horus, Mortarion, Angron, Abaddon, and Garviel Loken.