Megarachnids

The Megarachnids are a sentient race of xenos resembling giant spiders that resided on the planet Murder, which was also known as Urisarach and One-Forty-Twenty. An extremely hostile alien species, they were an old enemy of the non-Imperial human Interex civilisation who instead of commiting xenocide had exiled the surviving members of the species to the world they called Urisarach as an act of mercy. The Megarachnids were stripped of their instellar travel capabilities and weather control devices that created powerful atmospheric disturbances were constructed by the Interex across the planet to deter vessels from landing on Murder. A campaign of extermination was waged against them by the forces of the Imperium of Man many years later during the Great Crusade after several companies of Space Marines were lost to the Megarachnids' latent hostilities during the initial Imperial investigations of the planet. The war proved costly in Astartes lives, but seemed to be making steady progress by the sixth month. The campaign came to an abrupt end when the Interex arrived to see who was disturbing the planet and first contact occurred between them and the Imperium, as represented by the Warmaster Horus. Whether or not the campaign was continued after the destruction of the Interex is unknown to Imperial records, and the ultimate fate of this species is unrecorded in existing Imperial records.

Physiology
Megarachnids had eight legs, with four legs used for locamotion and the other four ending in sword-like claws. Their entire bodies were covered in an organically-produced metallic substance that made their claws so hard that they were able to easily cut through the Ceramite of a Space Marine's Power Armour, and their hides proved resistant even to Bolter rounds. When this metal exoskeleton was breached, the Megarachnids bled grey blood and had pink flesh contained within.

Megarachnids had no visible eyes, but were somehow able to find prey and organize their attacks. All communication was done through clicks made from their mouths, which were described as razor sharp metallic pincers able to tear through Ceramite just as easily as their blade-tipped claws. Despite these commonalities, the Megarachnids came in a wide variety of shapes and sizes. The forces of the Imperium encountered Megarachnids that had wings, the ability to spin silk and spit cement, and enormous Megarachnids almost the size of Titans who likely played the same role in their forces. It also appeared that the larger the Megarachnid was in size, the more authority it had over its fellows.

Much of the surface of Murder was covered in forests of giant grass stalks the size of towering trees. At the base of virtually every one of these grass stalks were undulating purple sacs that drained fluid from the plants. These sacs made a constant hissing sound, and were believed to be the eggs of the Megarachnids. The eggs were so numerous that after the landing parties started destroying them for hours it was later decided to be a futile effort.

History
Murder had been intended by the Interex to serve as a reservation for the Megarachnids, a world where they could live without molestation and also be unable to vent their innate hostility on any other galactic civilisation. Things stayed this way until the final years of the Great Crusade in the 31st Millennium, when the 140th Expeditionary Fleet commanded by the Blood Angels Legion's Captain Khitas Frome discovered the planet. Unable to translate the warnings from the orbiting Interex satellite beacons that warned approaching starships to stay away from Urisarach since it was a prison world, the Blood Angels Captain Khitas Frome ordered the fleet's entire contingent of three companies of Space Marines to begin landing operations to investigate and bring the planet into Imperial Compliance. Due to the extreme atmospheric turbulance, all of the Blood Angels' shuttles attempting to land on the planet became scattered and were thrown far off-course which left the Imperial landing parties isolated from each other. The atmosphere also affected Vox (radio) communications and made it difficult for the Imperial forces to coordinate their movements. Ground teams soon started sending garbled transmissions to the fleet's ships in orbit, reporting that the planet was inhabited by extremely hostile xenos. As the reports continued, the xenos were described as too numerous and formidable to defeat without reinforcements. Not long afterwards, the Blood Angels made urgent distress calls requesting immediate reinforcements and extraction. The last transmisson recieved by the fleet came from Captain Khitas Frome himself, who noted through clenched teeth, "This. World. Is. Murder." The name stuck and became the Imperium's informal appellation for Urisarach, which was officially designated in Imperial records One-Forty-Twenty as the twentieth world reached by the 140th Expeditionary Fleet.

Murdering Murder
A company of the Emperor's Children Legion arrived in response to the Blood Angels' distress calls. They made the same mistakes the Blood Angels had made, and had their landing parties scattered by the planet's powerful atmospheric disturbances. While the company took heavy casualties, one landing party led by Captain Saul Tarvitz discovered a large rock-like structure that resembled a dead tree. This "tree" had the bodies of several Blood Angels Space Marines impaled on its many branches with flying variants of the Megarachnids feasting upon the bodies. After destroying the structure, the sky above where the "tree" had been suddenly began to clear of the violent storms that had afflicted the Imperial forces from the beginning. Captain Tarvitz realized that these structures were actually artificial weather control devices that were responsible for Murder's heavy atmospheric turbulance.

The Megarchnids immediately went about rebuilding the "tree" and sent hundreds of warriors to slaughter the last few Emperor's Children remaining. But just as the Space Marines were about to be overwhelmed, a relief force of newly-arrived-Luna Wolves Astartes from the Warmaster Horus's own 63rd Expeditionary Fleet began to land through the breach in the atmosphere. The Megarachnids were scattered and a full-scale assault on the hostile xenos of Murder began in earnest.

Ten companies of Luna Wolves, the remnants of the Emperor's Children, tens of thousands of Imperial Guardsmen, and several Legio Mortis Titans proceeded to level entire swathes of the grass stalk forests and destroy every one of the "trees" they encountered, which steadily eroded Murder's atmospheric barrier. The Warmaster Horus, who was commanding the Imperialassault from his flagship in orbit, was very pleased with the progress being made. Some consideration had been paid to initiating a withdrawal from Murder now that a proper landing zone was available to allow an easy extraction of the troops, when an unexpected visitor suddenly arrived.

The Primarch Sanguinius came to the world to inspect the dead of his original Blood Angels landing force that had been wiped out early in the campaign. With tears in his eyes Sanguinius asked his brother Horus if he would join him in a campaign of vengance against the foul xenos. The Warmaster replied: "Yes, let us murder Murder."

Adding five of his companies of Blood Angels to the Imperial invasion force, Sanguinius and his forces fought alongside the Warmaster against the aliens. Thousands of Megarachnids poured out of the forests and canyons of Murder in an endless wave. Despite never retreating from the Imperial assault, the Megarachnids only continued to lose ground. By the sixth month of the campaign it seemed the Megarachnids would soon face extinction when the a fleet deployed by the Interex arrived in the star system to determine who had assaulted their Megarachnids' reservation world. Finding contact with the highly-advanced humans of the Interex to be a more pressing issue that needed to be dealt with, the Warmaster ended the campaign against the xenos of Murder. The Megarachnids had been saved from extinction for a second time by their old enemy. Imperial records do not indicate the final fate of this savage species once the Horus Heresy began.