Hrud
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The Hrud are a race as old, if not older, than humanity itself. They were a creation taken over from the old Rogue Trader days and into modern Warhammer 40,000.
Initially, they were the Space Skaven, and garnered little attention. Then along came Xenology, an out-of-print Black Library book which detailed the findings of an Inquisitor on his pan-galactic, semi-heretical travels to understand what makes aliens tick; mostly by probing, disemboweling, dissecting, and doing generic alien-abduction-type things to random xenos scum. In his travels, he encountered the Hrud, a jellyfish-like race of decaying spine-creatures, and that is how GW sees them now. The Hrud managed to avoid the ax, and skirted the edges of unpopularity to such and extent that GW actually gave them a face-lift as 40k became less Warhammer Fantasy in space and more grimdark, unlike their long-lost brethren the Squa- *BLAM*.
Origins
The Hrud are a nocturnal race of scavengers and parasites. They have a very strong connection to the warp, which manifests as an aura of darkness which surrounds the Hrud in question. Being near the Hrud is a thoroughly pleasant experience, as the field of darkness will age, decay and, pretty much wreck all nice things within the vicinity at a ridiculous speed. It is unknown if they are the chosen race of Nurgle, but the safe bet is that they probably are. Hrud can extrude a wide multitude of poisons from their bodies, and this eventually becomes a toxic miasma which hangs prevalent in the air of Hrud-infested areas.
Techno-Warpy Goodness
In a weird twist, Hrud are both technologically advanced and technologically backwards. While the average Hrud is covered in rags, and would rather hide in a dark corner than build a house, the Hrud race are fantastic at reverse-engineering and scavenging. Unlike the Orks, who can durr and hurr their way to a Plasma Cannon, Hrud actually know how things work. More than can be said of the Adeptus Mechanicus. Due to the lack of ammunition and repair facilites, the Hrud usually end up wielding whatever weapon they stole fused to their own technology (which looks more industrial, gritty and slimy than a Forge World).
When they don't want to steal something, they use what are called Fusils, which are warp-plasma muskets. The plasma moves between the Warp and realspace to bypass armor and shields. This same technology was used to make Warp Cores used by the squats. When the Mechanicus tried to tinker with the same technology, they DATA EXPUNGED a whole planet. Inquisitor Kryptman (no, I don't know who that is) claimed that Hrud Fusils are a synthesis of Melta and Plasma technologies and use a lance-like beam to chew through armor, but he probably mistook another Hrud weapon for the Fusil.
What the Hrud are up to now
Hrud commonly live in underground cities called Juunlaks, made of tunnels, dirt, and dark. Juunlaks are found near centers of population in the host civilization, but never in very large numbers. Because the Hrud could care less about what they leech on to, Juunlaks and/or Hrud infestations have been found on, but not limited to: Imperial Hive Worlds, Ork Worlds, Space Hulks, large Battleships of all races, the planet of Es'Tau, and Craftworld Saim'Hann. Eldrad was know to have eliminated the Hrud present on Saim'Hann, likely because he wanted to have the nice things on Saim'Hann that the Hrud would have made unhaveable.
Every so often, the Hrud somehow gather together into a huge Peh-ha, or mass migration. This is probably because they have become too populous in one place and must leave to avoid detection and, presumably, to get way from that one uncle who always wants to play Monopoly at family reunions. The irony is that once they leave, everyone and their grandmother steers clear of them. That many Hrud in one place can seriously mess you up, and in combat they are usually covered in shadow, and invisible until they're two feet away from you with a stolen pistol to your skull.
On planets, they mainly survive on petty thievery, power-sapping, and mostly being a filthy xenos parasite. They do get hired on occasion as assassins, but they never hang around long enough to be anything more than a reputable killer for one-off jobs.
See Also