So I'm still contemplating the Curse of Solomon, and have a few more ideas for what it could be.
So obvious candidates out of the way, a lictor, a mandrake, a daemon or a chaos champion, or the Beast, the foul xenos statted up as an example for the curse in the DH sourcebook.
Ideas I have: A mass haunting event, affecting much of the planet. Those who have died in darkness, in fear, in loneliness and so on aren't gone. They remain as an embodiment of the misery of the dying Hive World. Manifesting in the dark, as thick foul smelling liquid that drips from pipes, oozes from grates or bleeds from walls, which coalesces drop by drop into the undead visages of prior victims. These manifestations gather around their prey in great numbers, dragging them into the deepest dark, squeezing them into pipes too small for the human body, passing them through vents and grates, or pulling them body and soul into the walls and floor with the sound of cracking bone and bursting soft tissues. The curse cannot be removed from the world, though it can be defended against to a limited extent, with light, hope and courage.
An assassin. An agent of death and murder, adherent of a censured Death Cult native to Solomon which has largely been neutralised. Augmented and equipped to pass unseen in the dark, travel through tight spaces, and driven by the belief that Solomon can only be saved by the blood sacrifice of it's people to the Emperor. Bionic joints allow their limbs to contort, twist, compress and rotate, a bodyglove renders them into a sleek black shadow invisible to most scans, and implanted weaponry enables them to flense, pulp, break and shatter their victims into little more than slurry contained in a sack of their own flayed skin. .
A xenos predator placed there by the Beast House, implanted with bionics that record it's killings, driven by instinct to prowl in the dark and prey on the weak. Recordings of it's hunts are sold by the Beast House, or shared with their blood hungry patrons. I imagine a creature akin to a feline serpent, like a leopard stretched out, hunting people and taking their corpses to the high places of the lower hives to feed on them in secret.
An architectural murder machine. In the early days of Solomon it was found to be the site of a Dark Age facility, one that could not be moved nor dissasembled. The facility was built to process human remains into resources, creating a more palatable and nutritious version of corpse starch, as well as other resources. The Imperium was delighted to have such an asset, but as is often the case it was eventually lost. Forgotten in the deep dark pits of the world. But the facility hungers, it needs to produce corpse starch, to feed the swollen populace of Solomon. Programming in the ancient cogitator banks that rule the facility dictate that it must feed, so it may nurse mankind on it's own recycled remains. But Solomon's population is vast, so very vast, and the Imperium no longer feeds corpses to the facility, it must hunt and so it finds the wretched, the lost, the dead and the dying. Mysterious techology abducts the living as readily as the dead, feeding them into the hoppers of a vast abattoir, a network of machines that break down and reconstitute organic feedstock into resources, which are then dispensed through a network of dispensers, doling out a ration of food, fuel and crude textiles to the populace. An unsustainable cycle, drawing out the agonising end of the world.
Monstrous hope. Superstition, paranoia and desperation have driven elements of the impoverished masses of Solomon offer sacrifices to the Beast, but those sacrifices do nothing. People die, they dissapear, they dwell in pain and misery and their desperate attempts to assauge their suffering did nothing. Then someone, or several someone's decided to try and make people think it worked. If your prospective sacrifice to the darkness is found having died of dehydration still shackled to a pipe, then your sacrifice meant nothing. If the person is gone, lost to the dark and the unnamed horrors in it, it at least gives the hope that it might have meant something. So all across Solomon there are those who have looked into the eyes of their frightened children, held their lovers as they weeped for a futile sacrifice, and come to the conclusion that if there is no beast, then someone must take up that role to ease the burdened minds of those who cannot face the horror that is Solomon.