Second Battle of Nidalos

Nidalos saw several battles throughout its' history, yet most of them happened during the Great Crusade and the Horus Heresy. The very last battle occued in the late M41, between the native population, The Flawless Host chaos warband and armies of the Europan Planetary Federation, that arrived to help defend the planet.

Ghosts of the past
Ten millennia ago, during the Horus Heresy, a full battle company of Space Wolves clashed with the forces of the Emperor's Children over control of this world, along with millions of mortal men at both sides. The battle was fierce, with the number of casualties rising every day by several thousand, yet the Captain Ulfric of the Space Wolves and his enemy, Captain Barbathor of the Emperor's Children, clashed again and again in a deadly battle of attrition. Soon it was apparent that the Emperor's Children had an upper hand, so Captain Ulfric ordered a retreat of his legion back to orbit, leaving behind the Imperial Army and performed an Exterminatus upon this world by Viral bombing, denying it for everyone. After the Virus Bombs were dropped, the VIth legion left the world, leaving it behind to its faith.

The Flawless Armor
Captain Barbathor, at least by a rumor, was supposed to wear a unique, powerful armor forged by Fulgrim, Primarch of the Emperor's Children himself, that was given to him for his loyal service and combat prowess. By the same rumor as mentioned before, he was supposed to fall somewhere on Nidalos somewhere during the clash with the Space Wolves, and so Eleaxus the Flawless decided he must find and recover fallen captain's body and claim the ancient relic armor for himself. With this goal in mind, he gathered forces and sailed to Nidalos.

Naarians
At the time of the attack, Naarians had about 145'000 professional troops dispersed throughout the city-states. They were well equipped and trained, yet the average number of 8000 troops per city was nowhere near ideal to face an invasion of such force. In addition, they could mobilize about 6 million people who went through basic training and acted like reservists, although the number of available military-grade firearms was about half of that.

The Flawless Host
The Flawless Host arrived in full force, taking 4 full companies of Heretic Astartes, as well as two million of cultists, militiamen, and traitor guardsmen. The Astartes were battle-hardened veterans of countless battles and a significant portion of the traitor Guard saw some action as well. They were disciplined, well supplied and had some heavy armor with them. The cultists, whose number could have been anywhere between 1 to 1.5 million in total were mostly a horde of ill-organized, ragtag junkies high on every drug they managed to get their hands on, equipped only with primitive weapons, such as clubs, stub-pistols, and auto guns. The fleet the Heretics arrived in consisted of 1 battle barge, 3 grand cruisers, and 9 cruisers, along with countless escorts and transports.

Europans
Europans came in with most of what they got. Their fleet was comparable in size with the fleet of the enemy, led by the mighty battleship Spear of Orion, followed by 1st and 2nd Cruiser group, Carrier group, 2nd and 3rd Cruiser squadron, counting 11 large vessels in total, yet the real number would be close to 70, counting all the escorts and haulers. Speaking of land forces, Central Command approved the deployment of over 400,000 troops, all well trained, equipped and supplied, along with 4 full tanks regiments, 11 mechanized regiments, 37 infantry regiments of all types, and 12 artillery regiments, along with several bomber, and fighter squadrons. Adpetus Mechanicus arrived with a single Macroclade of the Skitarii from Ostrakan and even the Ecclesiarchy brought a Preceptory of Adepta Sororitas. Saint Beltaine herself took command of it.

Chaos Invasion
As the invaders attacked without warning and the first contact between them and Naarians was when the first dropships entered the atmosphere and painted long, fiery trails of fire across the skies, Naarians didn't have much time to react, and when the dropships landed near the Nulaar Zaar and bullets started flying, it was already too late. Nulaar Zaar was overwhelmed within several hours and all citizens killed. Naarian weapons and drilled tactics were soon found rather ineffective against hordes of rampaging cultists or armored columns of the traitor guardsmen, and both tanks and heretic Astartes were pretty much immune to all but the heaviest weapons the Naarians possessed. The only ones able to kill them were the War Priests by using their psychic skills. Many brave Naarians gave up their lives when they escorted a single War Priest on a suicide mission, to they could take a few of those armored giants with them to the grave. Within a month, 4 more cities fell to the invader's hands and within three, the entire world was overrun, but the escaped survivors waged a brutal guerilla war, using the terrain for their advantage. Thousands of traitors ended up being killed by the fungal spores, exploding plants or swallowed by the Desert Maws when they gave pursuit to a seemingly easy kill, that was, in fact, a well-executed ambush. Fortunately, the invaders weren't primarily interested in corrupting or exterminating the world as a whole, as their main goal was to find the ancient battleground and recover the relic armor. This gave the Naarians a bit more hope, but the death toll was already too high, with over 7 million dead, both soldiers and civilians alike. Luckily for the Naarians, help was already on the way.

Operation Showdown
When the Europan Strike Group arrived, they immediately formed positions and engaged the Heretics, first at long range with Nova Cannons but as most ships sat at low orbit, only those far enough from the surface were targeted to minimalize the collateral damage. Within the opening salvo, one Chaos cruiser was immediately obliterated by a direct hit by Thunderstrike, while two more were damaged. The Battle Barge of the Flawless Host returned fire with its Bombardment cannon, damaging Artemis, a Terrigon-class cruiser to a point she had to be evacuated.

Within days, the ships closed the distance to open fire from their main batteries. Ship formations passed one another, unleashing devastating broadside barrages while fighters and bombers chased each other to establish local superiority. However the longer the battle went, the less organized it became and soon the ships just circled one another, firing at whatever enemy they happened to stumble upon. Here and there a ship exploded or collided with one another, purposefully or not and despite the technological advantage the Europans had thanks to their up-gunned and partially automated ships, the enemy vessels imbued with Chaotic energies were a tougher nut to crack than it was presumed.

The operation climaxed with the battle of the giants, the Spear of Orion against the Herald of Sin, was one of the most important moments during the entire battle. As the battle went on, the two flagships ended up locked in a duel against one another. The two ships exchanged one barrage after another, while the smaller ships circled around in a seemingly chaotic dance of death. The duel lasted for several days and in the end, both ships were badly battered, leaving trails of smoke and debris in their wake, yet the crews refused to yield. The Herald of Sin, a corrupted Battle Barge was in a bit better shape than her opponent due to better armor and tougher superstructure, yet it had its engines damaged and the Spear of Orion was a bit more maneuverable. This also decided the outcome of the duel, as the chaos vessel got decapitated by a desperate, close-range Nova cannon shot directly under her bridge. The round punched through the damaged armor and the immensely powerful thermonuclear warhead it carried exploded within the ship, causing cataclysmic damage. The majority of the surviving crew died within seconds from the heat, shockwave, and radiation, and the blast itself torn the Battle Barge into several pieces, sending them flying in all directions. Several pieces hit the Spear of Orion, causing further damage to the armored prow and the Nova Cannon itself, rendering it inoperable.

After the gruesome destruction of the Herald of Sin, the battle for space superiority was over, with Europans being the winners, although the victory didn't come cheap. From all 11 large ships present, 3 were lost, 5 heavily damaged, and 2 lightly damaged. In addition, 17 escorts were lost, and 28 heavily damaged.

Operation Hammerfall
With the orbital superiority won, the Europans made a planetfall, landing at 4 different sites, each with one Battlegroup, always near a major city and secured the site. The first attack of the chaos forces came a few days after the Battlegroup Riegel made a planetfall, although it was easily repelled, as it was an ill-organized group of crazed cultists. In fact, this was a probing attack and the second attack came two days later, all with artillery preparation and spearheaded by both tanks and heretic Astartes, followed by overwhelming numbers of traitors and cultists alike. The battle was fierce and bloody. The Battlegroup Riegel got battered hard and had to endure for several days of near-constant heavy fighting until a relief in form of an orbital bombardment came in, scattering the heretics. Both sides suffered heavy casualties and the Battlegroup Riegel had to be withdrawn, as its strength was down by over 50%. This disaster was caused by the fact that the Europan troops were well trained, but only against other regular armies, not against hordes of violent cultists, let alone heavily armored Chaos Astartes.

The remaining three Battlegroups reunited with the Naarians and slowly pushed the heretics back, liberating several cities. Several larger formations of the enemy were encountered and eventually forced to retreat or destroyed. One such group was pushed out of the lush areas into the semi-desert with a large concentration of Desert Maws, where most of the heretics got devoured by the horrid creatures or died of dehydration and starvation later, all while the Europans held them pinned by sporadic mortar and artillery fire.

Conquering the cities, however, was a wholly different story. Mostly without the support of tanks and heavy artillery, soldiers had to the city street by street and room by room. Cultists and traitor guardsmen provided stiff resistance and often whole buildings had to be demolished or burned out, killing both the heretics and civilians that happened to be hiding inside. Soldiers also often found victims and sites of various tortures and ritual

Outcome
The Europans along with Naarians managed to repel the invasion, liberate all cities that were previously captured and completely destroy the enemy forces. It didn't come cheap, though, as the Europans only had about 130'000 casualties, most of which came from the Battlegroup Riegel, and twice as much came from the Naval Battlegroup in orbit. Casualties among the Naarians reach into millions.

Eleaxus the Flawless and the remaining Astartes were forced to escape the planet by performing a ritual to conjure a warp portal. He lost his entire group of warships and all of his mortal followers, along with a few dozen of his fellow marines, although he managed to find and recover the relic armor he primarily was after. We can only argue what had bigger value for him, the ancient and mighty armor or three million mortal lives.

Aftermath
When the invaders were definitely defeated, a great clean-up operation was launched. Bodies of the invaders were collected and burned in large pyres while Naarians and Europans were all buried in cemeteries or mass graves, but with deference and honor. Tanks and armored vehicles got scrapped and the hulls of destroyed chaos spaceships were hooked on a mag-tether and sent flying into the cosmic void.

With that dealt with, Europans brought hundreds of pieces of construction machinery, along with operators and specialists that helped to rebuild the destroyed cities. Also, Nidalos joined the Europan Planetary Federation, as it was a condition for the afterwar support.

Further fate of Eleaxus the Flawless is unknown.