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 anti-vaxers. 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! Unless you happen to have a stomach of fuckin' adamantium). 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 but 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 (by the way, when's the last time you talked to your mother?). 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.
They seem to dislike getting their proverbial hands dirty, eschewing open warfare in favor of stealth, manipulation, and the use of proxies. Their main agents within the Calixis Sector are the Amaranthine Syndicate, a subverted trade cartel that they use as a front for smuggling xenotech and instigating trade wars as well as far more malicious operations.
The Amaranthine Syndicate
The Syndicate starts their work by identifying someone in such dire straits that they'll accept help from anyone to get out of their current trouble, then appearing to that person with what looks to be a very generous deal. However, even as they alleviate the original problem the Syndicate works to make their "friend" increasingly dependent on the Syndicate's aid- their enemies are set against them, other forms of help become suddenly unavailable, and "accidents" occur with unusual frequency. When the subject is so heavily indebted to the Syndicate that they have no way out, they are taken to meet one of the "Principals" so the subject can better understand the bargain he's been forced into...and the consequences of trying to back out of the deal.
The Slaugth's demands are trivial or even nonsensical at first. They might ask for a book in a private library to be burned, or a commodity's price to be raised ever so slightly, or to reassign seemingly random personnel. It doesn't take long for their demands to escalate, however, and before long the subject will be ordered to taint medical supplies at their source, redirect relief shipments away from planets in need, or outright betray colonists to Slaugth raiders. Should they refuse, the Slaugth will eat them instead and find another desperate fool to take their place.
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, and haven't really seen mainstream attention in rulebooks. 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
However, the Slaugth (and Rak'Gol) do get a mention in Necromunda in the Book of Judgement, along with their weapons being available as black market wargear. So perhaps there is a slim chance for them to make an appearance on the tabletop after all. That being said, their fluff firmly establishes that they prefer infiltration and subterfuge over direct conflict to achieve their goals so they might not be a particularly good fit for having a full army anyway.
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 (Emperor rest his soul) 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 Xenocides fought by the Space Wolves and Dark Angels, and possibly where one of the two unknown legions was destroyed.
In the short story Living Relics, the Consecrators Chapter find one of these aboard a First Legion warship sent to battle them as part of a larger fleet, albeit one lost and found deep beyond the Eastern Fringe, far from the Rangdan strongholds in the galaxy's northwest.
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. However, in the book Alpharius: Head of the Hydra it is made quite clear that the Slaugth and the Rangda are two separate forces, they had entirely different void ships and modes of war: The Rangda's viciousness was compared to Orks, while the maggot-men of the Slaugth repulsed the Primarch on a very fundamental level. If Alpharius is to be believed (read: he's not), they operated in the same warzones so it is possible that the Slaugth and Rangda were close allies or related in some other way. Alpharius wonders if the Slaugth merely followed in the wake of the Rangdan destruction like vultures, or had driven them from worlds that they had claimed, or were able to direct Rangda towards the rich feeding grounds.
However, the FFG games also establish quite clearly that the Slaugth, for a couple of reasons, simply do not have the numbers to assault the Imperium head-on. And this is one Sector we're talking about. As much as the fans want to see Slaugth versus Custodes action, it is not going to happen.
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.