Enslavers
"A buey viejo no le falta garrapata. - There is no lack of fleas in old oxen."
- – Old Spanish saying
The Enslavers (also known as Psyrens, Krell, Dominators, Puppeteers and many other names by the denizens of the galaxy) are a strange "organism" that populates the Warp. We say that it's an organism, but really describing what these things actually are is an exercise in futility, considering their supposed origins and "biology". We can say, however, that they are some of the most dangerous things that can come out of the Warp, and considering the amount of horrifically dangerous shit populates that the Immaterium, that is quite an understatement.
"Origins"
Trying to explaing the origins of the enslavers forces us to go back to the days of the War in Heaven. During those times, the C'Tan and their necron slaves were beating the Old Ones and their allied races into submission, slowly but surely driving them into extinction. In their desperation, the Old Ones started creating more new races to try and stop them from going the way of the dodo, with little to no avail. This rapid creation of sentient species, alongside the tide of feelings of OCTILLIONS of beings embroiled in the war altered the properties of the Realm of Souls quite a bit, and it slowly but surely ended up altering its very nature, turning it into the hellish reality known as the Warp. It's at this point where the first enslavers start appearing, to the absolute horror of EVERYONE and EVERYTHING.
It is not known whether the enslavers were the first daemons, birthed from the emotions of all of this death and destruction, they were just organisms inhabiting the Warp and got attracted to the galaxy in a similar way the tyranids did (with a hunger for souls instead of flesh) or they were peaceful warp creatures horribly warped and deformed into the abominations they would become. In any case, they invaded the galaxy in millions, and basically singlehandedly stopped the conflict by murdering everyone involved.
These entities arrived just at the time where the Old Ones were barely avoiding extinction by the skin of their teeth against the C'Tan, who had been betrayed by the Necrons and started turning against each other. That small moment of respite turned out to be the calm before THE storm of the epoch, getting completely blindsided by this out of context problem. As such, the Old Ones and their slave species were utterly erased from existence, used as soul-fuel for the Enslavers. Combining the state of utter exhaustion with the quickness of the enslavers reproductive cycle, and most of the life in the galaxy was eradicated. The Necron and the C'Tan were more or less safe from these things, due to them not having souls anymore, but the C'Tan were getting shattered here and there and the necrons, seeing all of the clusterfuck the galaxy had become, decided to go "fuck it" and went to sleep for around 60 million years. The only known species that we 100% know that survived are the eldar and the orks: the eldar probably using the Webway as a refuge until the storm had passed, and the orks survived by... well, being orks, they probably managed to survived by putting spores in places not even the Enslavers could reach them. The Necrons also survived, but counting them as "species" is also a rather complicated debate, considering the necrontyr are definitively extinct, and in their place they left the automatas known as necrons. Anyways, once they managed to leave the galaxy clean of important life, they either went back to the Warp or they starved to death, leaving the way for the galaxy to recover from the cataclysm. With the millions of year passing, the memories of these creatures faded more and more, and by the time the Imperium of Man was created, those who still remembered them thought them to be truly gone...
However, this is 40k, and if something can ruin the galaxy, it will make its way to it. The enslavers have appeared from time to time to feed upon sentient species once again, although these ones are not nearly as powerful as those from the days of the Old Ones. Well, that or the sheer blind fanatical faith of the Imperium and its effect on the Warp makes them a relative non-issue. The Calixis Sector was victim to an enslaver infestation in 260.993.M41, however it was repelled by the forces of mankind. Nowadays, the Inquisition keeps an eye on these abominations, considering that a quick response is the key to keep them at bay.
It is also believed that Trazyn the Infinite has a carcass of an Enslaver in his collection. Considering how dangerous these beings are, as well as Trazyn's somewhat lacking security in regards to live specimens (remember when he tried to make a zoo of tyranids, only for them to escape and run amok in the planet?) it is good to know that the thing is actually a dessicated corpse... or at least, we believe it to be so...
"Biology"
Now, the first and most important thing to discuss regarding the enslavers' biology is to know whether they are daemons or not. Considering their nature, composition, behaviour, feeding habits and common habitats... We don't know. Really, if they are daemons, they share so many characteristics with regular organisms we could be forgiven to think of them as living beings, and viceversa. Both the Ordo Xenos and Ordo Malleus keep track of these things, just in case the other is wrong. At the very least we know they are older than Chaos, so they at least don't work alongside the Ruinous Powers (after all, if all the believers are eaten, there is no more worship emotion, so they would go kaput be weakened and only exist on aspects of reality such as change and decay...so really just Tzeench and Nurgle would survive). They may as well be the equivalent of tyranids for the Warp, a menace that came from outside the Immaterium (or at the very least a region of the Immaterium corresponding to another galaxy) to feed upon it, in which case, that's another apocalyptic threat the galaxy has to deal with. Great.
Physiology-wise they take a number of forms, but the most common aspect is something akin to a mix between a flea, a jellyfish and an octopus, all bloated into ridiculous proportions (they usually have more or less the size of a human, though). They move by floating in the air, probably using their psychic powers to levitate. They use their numerous tendrils to search for and feed upon the souls of the living, in particular psykers. Once found, they nail their tendrils inside of the victim's brain and start sucking out the poor sod's soul, while simultaneously turning it into a drooling slave, ready to fight for the Enslavers, defeating potential threats and searching for more prey. Once the victim is more or less done, they will put their eggs inside the target's brain, only for them to hatch a year or so later, making its head explode into more Enslavers. Sufice to say, if this process is not stopped immediately, an Enslaver infestation can destroy planets in a matter of decades, though they will try to be as stealthy as possible. And by the way, we don't even know whether they are a sapient species or not, so this behavior might be instinct or just a plan hatched with the borrowed intelligence of their host. Again, very little information for us to just corroborate anything major. If the Enslavers can't get any food, they will either return to the Warp or starve to death.
Presence in the Tabletop and Vidya
Enslavers come from the early days of Rogue Trader, although most of their fluff comes from later works. Tabletop-wise, they were given rules in the Chapter Approved Creature Feature, although no models were made for them. If there are any homebrew rules for this thing, someone add them here, please.
In Warhammer 40k Gladius, they appeared as a neutral faction that spawned randomly which they would attack players and computers alike. Living up to their names, they are capable of psychically enslaving units and turning them against their former allies (cept for the Necrons, for obvious reasons). Killing the enslavers will free the mind-controlled units from their power.