Megarachnids

Megarachnids are a sentient race of xenos resembling giant spiders that reside on the planet Murder (aka. Urisarach, Spiderland, One-Forty-Twenty). An extremely hostile race, they were an old enemy of the Interex civilization who instead of commiting xenocide exiled them to Murder as an act of mercy. The Megarachnids were robbed of all instellar travel and devices that created powerful atmospheric disturbances were constructed across the planet to deter ships from landing on Murder. A campaign of extermination was waged against them by the Imperium many years later during the Great Crusade after several companies of space marines were lost during the initial investigations of the planet. The war was costly, but seemed to be making steady progress by the sixth month. The campaign came to an abrubt end when the Interex arrived to see who was disturbing the planet. Whether or not the campaign was continued after the destruction of the Interex is unknown, but with the knowledge that Murder was the only planet the Megarachnids possessed it was possible they were simply ignored by the Imperium as the threat was already contained.

Physiology
Megarachnids were described as having eight legs, with four legs used for locamotion and the other four having sword-like claws at the end. Their entire bodies were covered in a organic metallic substance that made their claws able to easily cut through the ceramite of a space marine's power armor, and provided resistance to bolter rounds. When this metal exoskeleton was breached, the megarachnids bled grey blood and had pink flesh inside.

They had no visible eyes, but were somehow able to find prey and organize their attacks. 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 sword claws. Aside from these common traits they all shared, Megarachnids came in a wide variety of shapes and sizes. The Imperium encountered Megarachnids that had wings, the ability to spin silk and spit cement, and enormous megarachnids almost the size of titans. It also appeared the larger the megarachnid, the more authority it had over the others.

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 draining fluid from the plants. These sacs made a constant hissing sound, and were believed to be the eggs of megarachnids. The eggs were so numerous that after the landing parties started destroying them for hours it was decided to be a futile effort.

Murder's Namesake
Murder had become a reservation for the Megarachnids, where they could live without molestation and be unable to cause any other races harm. Things stayed this way until the Great Crusade, when the 140th Expeditionary Fleet commanded by the Blood Angels Legion discovered the planet. Unable to translate the warnings from Interex beacons telling ships to stay away, the Blood Angels Captain Khitas Frome ordered the entire contingent of three companies to begin landing operations to investigate and bring the planet into Imperial compliance. Due to the extreme atmospheric turbulance, all Blood Angel shuttles attempting to land on the planet became scattered and thrown far offcourse leaving the landing parties isolated from eachother. The atmosphere also affected vox communications. Ground teams soon started sending garbled transmissions to the ships in orbit, reporting the planet was inhabited by hostile xenos. As reports continued, the xenos were described as too numerous and formidible to defeat without reinforcements. Not long afterwards the Blood Angels made urgent distress calls requesting for immediate reinforcements and extraction. The last transmisson recieved came from Captain Khitas Frome himself: "This. World. Is. Murder."

Murdering Murder
A company of the Emperor's Children Legion arrived responding to the distress calls. They made the same mistakes the Blood Angels made, and had their landing parties scattered by the atmospheric disturbances. While the company took heavy casualties, one landing party led by one 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 been troubling them from the beginning. Captain Tarvitz relized that these structres were what was causing the massive 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 began to land through the breach in the atmosphere. The Megarachnids were scattered and full scale assault on the hostile natives began.

Ten companies of Luna Wolves, the remnants of the Emperor's Children, tens of thousands of Imperial Guardsmen, and several Legio Titans leveled entire swathes of the forests. Destroying every one of the "trees" they encountered. Steadily expanding the hole in Murder's atmospheric barrier. Warmaster Horus, who was commanding the Imperials from space, was very pleased with the progress being made. There was some consideration to simply leave Murder now that a proper landing zone was available to allow easy withdrawal of the troops, when an unexpected visitor arrived.

Primarch Sanguinius himself arrived to inspect the dead of the wiped out Blood Angels landing force. 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 invasion force, Sanguinius and his forces fought alongside the Warmaster against the aliens. Thousands of Megarachnids poured out of the forests, canyons, and seemingly every crack of Murder in an endless wave. Despite never retreating from the Imperials the Megarachnids only continued to lose ground. By the sixth month of the campaign it seemed the Megarachnids would soon face extinction when the Interex fleet arrived in the system. Finding the Interex to be a more pressing issue to focus on, the Warmaster ended the campaign. The Megarachnids were saved for a second time by their old enemy.