Megarachnid

The Megarachnids were a species of insectoid alien which featured in the Black Library novel Horus Rising, which was itself set in Games Workshop's Warhammer 40,000 universe.

Description
As their name might suggest, the Megarachnids bore a certain resemblance to terrestrial spiders, although they were far greater in size, with normal specimens standing taller than an Astartes in power armour and the largest reaching dozens of metres in height. Their rear two pairs of limbs were their primary method of locomotion, whilst the frontal pairs were devoted to attack, being equipped with long, scything talons sharp enough to slice through power armour with ease. Their bodies appeared to be coated in some kind of organic metal capable of shrugging off multiple rounds of explosive ammunition, and making their talons even deadlier in combat. Whilst the Megarachnids apparently lived as a group of social creatures, with several different castes being identified, they did not seem to have a ruling or queen caste, although the largest of them appeared to be of greater importance to the gestalt than the smaller ones.

The Megarachnids were also capable of excreting a form of bio-concrete from their abdomens which, while being useful in close-quarters combat, was primarily used for the creation of the strange techno-organic trees that dotted their world's surface. These constructs were utilised to achieve a variety of effects, from scrambling electromagnetic communications to weather control on a planetary scale.

History
The Megarachnids were once a spacefaring race who possessed a small empire in an unknown sector of the galaxy, but long ago fought a brutal war against another empire composed of humans and xenos, known as the Interex, for unknown reasons. The Megarachnids eventually lost this war, but as the Interex were a relatively moral faction by Warhammer 40,000 standards, rather than exterminating their foes, they removed all forms of interstellar travel from the Megarachnids and confined them to the world which would become known as Murder to the Imperium of Man, leaving a series of beacons in orbit to warn others of the dangerous species present on the planet.

During the Imperium of Man's Great Crusade to reclaim the galaxy, a small unit of Blood Angels Adeptus Astartes chanced upon Murder (initially designated One-Forty-Twenty), and descended to the planet's surface. Subsequently, all contact was lost with the group, save for a single distress signal sent by the leader of the Blood Angels force, which simply said "This. World. Is. Murder." The planet was later renamed Murder in recognition of these warriors.

In response to the signal, several much-larger Expeditionary Fleets, from not only the Blood Angels but also a range of other Astartes Legions and Imperial Army units, responded and launched a campaign of annihilation against the Megarachnids, which had wiped out the initial landing force. Although the action lasted six months and was an extremely hard-fought battle for the Imperials, it was eventually successful, and the Megarachnids were hunted to extinction.