Remember when you first started playing 40K and thought it was all sunshine and rainbows? Sure, tyranids are pretty scary, and their Genestealer cult babies are works of art that would make H.P. Lovecraft blush. The Warp is literal nightmare fuel, you say, but the warp is just full of people who were neglected by their parents, Polly after cutting crackers out of his diet, grumpy puppies, naked ladies, and Florida residents. But that just wasn't enough. And then a certain fluff writer favored us with these things...
Like a fever nightmare that crawled out of the dreams of someone who has PTSD from when they had jiggers on them (Don't look them up please dear god!). The Slaugth are a literal swarm of maggots that eat dead bodies (Guess how they get the bodies dead in the first place) although their favorite part of the body to chew on are the brains. When the Slaugth do eat brains they actually absorb its information and memories to find new places to inhabit and new victims to devour. It's also heavily implied that they get high off of this.
The Slaugth are aliens that look like three-meter-tall swarms of maggots, dripping with necrotic mucus. Their nature means that they can stretch and reform their bodies at will, and move with an "obscene, boneless fluidity". They can also regenerate very quickly from all by the most catastrophic injuries. When they want to pass among humans, they put on shroud robes and use masks to hide their 'beautiful faces'. Because of their nature, they tend to stay on the fringes of known space or lurk in obscure places, such as deep within the lowest parts of hive worlds or your mom's basement, popping out long enough to OM-NOM some unfortunate humans before slithering back off into the darkness. They've apparently been around for a really long time, but they're so damn good at staying hidden that no one knows how old they actually are, what their ultimate goal is, or even the location of their homeworld. (Some of the RPG fluff seems to imply that the Inquisition has standing orders to shoot anyone who runs into a Slaugth, which wouldn't help matters any.) They also like to give their weird-ass tech to greedy rogue traders and planetary governors as a way to cause political disarray and to soften up the local Imperial Guard and PDF so they can go to town on the local populace without anyone noticing.
Nowadays the Slaugth and their fans are praying to Yog Sothoth that they get their own codex, but the Slaugth, along with other minor races such as the Rak'gol and the Enoulians were the brain-children (pun intended) of Fantasy Flight Games, which puts their canon status into question. Not to mention the whole Worm That Walks thing is a long established horror trope that has been used in other RPGs before. Chances are that it will only happen when Games Workshop truly runs out of
Possible Crusade-Era Connection
The Slaugth are alluded to several times across the Horus Heresy series, which makes sense considering that Forgeworld fluff guru Alan Bligh was one of the lead writers of Dark Heresy, so might be indirectly using his big bad worm monsters as the terrible boogeymen from the galactic north and east that are only spoken of in whispers.
In one of the Forge World books, a War Hounds character, prior to Angron's return to the legion was critically injured by the "Slaugth Murder-Minds" at Rangda, which connects the Slaugth to the Rangdan Xenocide campaigns which were fought by the Space Wolves and Dark Angels and possibly where one of the two unknown legions was destroyed.
They are also mentioned as the discoverers and keepers of a young Alpharius in one of his spurious origin stories, but This Account Is A Lie (but may have contained grains of truth). As the Slaugth are known to be master infiltrators and frequently use humans to spy on their behalf, this raises some disturbing questions as to how he learned his infiltration skills and what the Slaugth planned to do with him.
The Rangda were described as being so powerful they nearly killed the Imperium stone dead even with the Emperor alive and kicking at the helm (Something no other foe has ever been able to do), and the Emperor was forced to unleash the Labyrinth of Night to defeat them. If the Slaugth are the Rangda, then if they ever returned in force nothing would be able to stop them from buttfucking everything in their path.
Despite their appearance, the Slaugth are a highly advanced race and possess technology on par with even the Necrons. Their specialty is biomechanical technology, aka technology with organic parts, so instead of training troops they essentially farm them and then stick them full of robotic parts if they didn't have them from the get-go. They may or may not have zombie minions as most of their bio-mechanical constructs are usually constructed from the remains of others.
One of the constructs they have is called a Harvester Construct, which is a floating ball of flesh with a robotic arm that goes around and decapitates people and preserves their heads so the Slaugth can eat the brains later. Whoever comes up with these bizarre ideas must have been smoking some strong ass drugs. Better yet whose brains were the Slaugth smoking?
The second one we know about is called a Warrior Construct. which unlike the others has a shape and have an uncanny resemblance to human genitals. On the other hand, the Warrior Construct also looks like sea anemones with legs and teeth as the description given to them are, "These pale, oval masses of fungal flesh are filled with purple veins and metal lattices." They also have necrotic beamers and have bone blades coming out of their tendrils. So in other words it's a biomechanical attack penis which is the Slaugth's amalgamation containing all of 4chan. Or they just found H.R. Giger's art collection
Their clothing of choice and pseudo-armor are called Shroud Cloaks. These ragged, tattered robes absorb and refract light and seem to move as if they were alive, making them look like misshapen humans in the distance. They also have Shroud Armor, which is a different tier of shroud tech which actually clouds the perception of anyone observing the Slaugth wearing it, effectively making them invisible.
For weapons the Slaugth possess "Necrotic weaponry", which appear in a variety of forms. They have Necrotic Scepters, which can be used as a melee or ranged weapon. The warrior constructs mentioned above have a Necrotic Beamer that can can turn organic targets into dust in seconds, and high-ranked Slaugth get a Necrotic Lance that can blow up main battle tanks.