The folk of the deep, long forgotten and through their own efforts kept secret. They are conniving, pure evil, serving the Ruined Gods. They have eldritch knowledge, with strange and truly twisted creatures in their service, and that they too serve. They are the purest and most nightmarish of evils. For a centuries it was believed they'd been wiped out during the Sundering, perhaps admist all the infighting amongst the different factions and species that worship such dark things, like the long forgotten ratfolk. But that is far from the truth and the horror has returned in recent decades, and is a known danger now.
Long forgotten, the horrors of the merfolk have recently been rediscovered and have returned. Gills and fish like scales but in a humanoid like form, with strange magicks and weapons, commanding all manner of nightmarish beasts of the deep, the Merfolk do know fear and caution now, for they have no answer to smokepowder weapons if they ever get in a pitched battle above the surface of the water. They have no real concept of ranged weaponry, simply due to their environment, their conceptualization of such things are far more limited in their distance of engagements than those whom live above in the realm of air and land. Water is not condusive to ranged weaponry after all. After the era of the Sundering, and during it, the mer did not fair well in competition with the other beasts of darkness, and swiftly were once again forced to exist more or less only at and in the depths of the sea, for they simply could not hold coastal towns or dry land of any kind against species more adapted and tactically capable on such terrain. However despite the initial belief held by those whom eventually reclaimed society and their homelands, the perception that the merfolk had been wiped out was far from the truth.
In recent decades, that false belief has been rudely shattered, smaller coastal villages and towns disappearing overnight on occasion, ships simply gutted and set adrift to float into a port, the bodies of those whom were mercifully just killed, left as evidence for when the dead vessel eventually just floats into a port. However they also take prisoners, the mucus some of them produce capable of removing one's ability to breathe air, but making them as a fish, able to breath within the waters of the sea.
The fates that await these prisoners is one of true nightmares, for those of the Mer worship Invidias and Libidine primarily amongst all the Ruined Gods. They seek to torture, to enjoy the exquisite taste of all manner of heinous excessive delights from your slavery, and to use you to breed their own species, but evolved. Over time and the various exquisite tortures and fleshwarping horrors they will put you through, they will change you, alter you, turn you into something more, but also something less. Eventually, the being taken prisoner will no longer exist, your mind utterly shattered, ruined from a decade or two of such horrifying delights, your body forcibly changed, appendages altered, organs changed or replaced, to create creatures capable of breeding with the merfolk, creatures with desired traits, as well as creatures to be used as monstrous raiding fodder for future slave taking trips. All the while, though your personality broken, your body changed, your spirit, your soul, still exists within, able to feel every exquisite suffering and violation of what was once your body, your spirit entrapped by the darkest and most foul of Ruinous Magicks, chained and kept alive and aware enough by those magickal forces to experience it all, but weak enough to be unable to do anything about it. It is unknown how long a soul in this state will last, but it is assumed it cannot last forever, and will eventually be extinquished by such horrors.