The Megarachnids were a fearsome and violent Xenos race, resembling giant spiders and winged insects.
In the distant past, the Megarachnids fought a terrible war against the Interex but were ultimately defeated. Rather than exterminate their foes, the benevolent Interex exiled the Megarachnids to the planet Urisarach and denied from them all means of interstellar travel.[Needs Citation]
During the Great Crusade, the 140th Expeditionary Fleet discovered Urisarach (dubbed "One-Forty Twenty"), and, ignoring the Interex's orbital warning beacons, descended to the surface to effect compliance in the name of the Imperium. Then all contact with the 140th ceased, save for a lone distress call from Blood Angels Captain Khitas Frome, which ended "This. World. Is. Murder".[1x]
The Imperial forces that responded to the distress call included the 63rd Expeditionary Fleet under Warmaster Horus, a detachment from the Emperor's Children under Lord Commander Eidolon, and a relief force from the Blood Angels, led by Sanguinius himself. The War on Murder as it was dubbed, took several months, and all three forces suffered serious losses, but eventually the Megarachnids were exterminated.[Needs Citation]
The war on Murder inadvertently led to the Imperium's first contact with the Interex, who arrived shortly after the war ended, in response to signals from their orbital beacons, and rebuked the Imperials for ignoring their warnings.[Needs Citation]
Biology and Ecology
The Megarachnids possessed long sword-like claws that appeared to be made from an organic metallic substance, and lived and worked in large swarms.
Their bodies were heavily augmented through a technological or biological process that, according to Captain Saul Tarvitz, made it impossible to discern where their bodies ended and their weapons began.[Needs Citation] His fellow Captain, Lucius, appropriated a severed Megarachnid's claw as a second sword, but its edge was so keen that he cut his hand just by holding it in his ceramite gauntlet.[Needs Citation]
The Megarachnids grew forests of "trees" from a hard, stone-like substance all over the planet's surface. The significance of these trees was not apparent to the Imperial forces until Captain Tarvitz insisted on destroying one, and discovered that the trees were generating the electromagnetic radiation that disrupted vox communication and drop pod telemetry, and also caused severe weather storms that confounded the attacking Space Marines.[Needs Citation]