Hrud
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 a Magos Biologis 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 itself 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.
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 Hrud Migration.