Remember when you played 40k and thought it was all 'sunshine' and 'rainbows' yea tyranids are mean and their Genestealer cult babies are works of art that would make H.P. Lovecraft blush. "Ooh but the warp is literal nightmare fuel" you say, yea but the warp is just full of people who were neglected by their parents, poly after cutting crackers out of his diet, grumpy puppies, naked ladies, and Florida residents. Nothing of real fright...and then these fuckers get written into existence.
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 swarms of maggots wearing tattered black robes and to make them perform a hostile takeover of your dreams tonight, uses masks to hide their 'beautiful faces'. Because of their nature, they like to stay hidden in obscure places like 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 politicians and governors as a way to cause political disarray and to soften up the local Imperial Guard and PDF.
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
In all honesty, with all the obscure factions like genestealer cults and skitarii getting their own armies it wouldn't be surprising if the Slaugth get some minor cameo in one of the armies. The Calixis Sector and the Koronus Expanse are being obliquely mentioned in the rulebooks these days. So who knows? More likely, better known and fluff embedded races like the Hrud or Demiurg will get a codex first.
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, which is admittedly false but may have contained grains of truth. (The sheer fact that they didn't down right eat him if this is true is horrifying, makes you wonder how his legion developed their infiltration.)
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.
Well, Slaugth own the patent for disposable diapers as anyone who stumbles upon them will instantly shit their pants. However, despite their appearances the Slaugth are a sufficiently advanced race with tech 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 it 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 that has a robotic arm that goes around and decapitates people and preserves their heads inside said floating ball of flesh...for some reason....yeah...they have flying ball sacks floating around decapitating people. Whoever has the brains to come 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 the Shroud Cloaks, these rags absorb and refract light and are alive in a sense as it 'moves and writhes', making them look like misshapen humans in the distance.
Shroud Armor is a higher tier of shroud tech which actually clouds the perception of them, it's literal a robe that projects a mind rape field. Pleasant.
For weapons the Slaugth have some downright disturbing weapons more horrible than anything the tyranids can throw at ya. "Necrotic weaponry" are essentially guns that either rot away the flesh, or turn it to dust, the sources are unclear, but both ways are horrifying.