There are many foul beings that inhabit the Warp. There are the sinister Enslavers that can bend and entire planet to their will. There are the Dridect that seek order, but can only bring about insanity, and of course the numberless Daemons that serve the foul Chaos Gods. However the Warp is also called the Sea of Souls because the spirits of every mortal creature that has a soul goes there when their mortal coil is shed. While many of these spirits wander the Warp and trouble the living no longer some direct their minds towards the Materium. Among the more malevolent of these spirits are a truly frightening group of monsters called Wight-Fiends. Also called the Rancorous Dead, these former spirits of the living have transformed into nightmarish monsters filled with an unending hatred for the living, and indeed life in general. These ghoulish aberrations would end all life if they had their wish. For life is cruel and unfair, but there is peace and justice for all in the grave, or so they tell themselves.
The Wight-Fiends are spirits of the dead turned monstrous by a combination of foul sorcery and spite. Each and every Wight-Fiend cares only for one thing. To bring death and suffering to the living. A task that they set to with relish. To understand why a Wight-Fiend wishes to slay the living it is beneficial to understand what kind of souls end up becoming Wight-Fiends.
Among the mortal races strife and sorrow are facts of life. It is a blessed person indeed that never feels any kind of misery in their lifetime. However amongst mortals there are those that suffer more than others. People whose every waking moment is filled with misery and pain. The circumstances behind this pain vary greatly from individual to individual, but all have one thing in common. For there unfortunate souls life holds no joy or value. For them it brings nothing but pain. To these individuals life is a cruel thing, offering nothing but the promise of fresh agony in the days to come. Often these individuals are those that are denied even the simplest pleasures of life, or are otherwise unable to appreciate them.
These individuals inevitably seek one of two things if not both. The first is release. A way to escape the living Hell that they are forced to endure. Invariably the only way to attain this escape is through death. The other desire of these persons is revenge. This is sometimes directed at a specific source for which they blame their misery, but often these bitter souls look at life and the world in general as their tormentors. To these individuals bringing death to others is a mercy, sparing them from the tortures of life, or even a form of vengeance on life itself. It is from both of these pools of individuals that Wight-Fiends are drawn from.
Wight-Fiends seek out both the living and the dead that have been tormented by their lives. They then whisper to these individuals, throwing into stark contrast the unjustness of their suffering, nurturing their bitterness and spite for life which has seemingly forsaken them. In the case of the dead they then twist those willing to embrace these feelings into new Wight-Fiends. In the living however the Rancorous Dead often take a more measured approach. They often coerce these individuals into murdering and terrorizing their fellow mortals. Wight-Fiends have even been known to form Death Cults dedicated to ending all life and seeking release in death. To those mortals that the Wight-Fiends find to be worthy they will teach and ritual. This ritual allows one to shuffle of their mortal coil and join the Wight-Fiends in the Endless Grave.
Wight-Fiends are obsessed only with killing the living. These cruel and jealous spirits despise life in all its forms. They were tormented individuals in their living years, and thus nurtured a hatred for life and for those who were blessed enough to enjoy it. To a Wight-Fiend the act of murder is not a sin. In fact they view it as a liberation and a balancing of the scales. In life many suffer, and a few are allowed to taste happiness and ease. This is the height of cruelty and injustice in their warped minds. Thus to send each and every living thing to the grave, where all are made equal and the strife of living is gone, is considered to be the ultimate act of justice in an uncaring world. In truth the Wight-Fiends are deceiving both themselves and others. They pursue their genocidal desires out of a rancor festering within their hearts, not to grant peace. Their jealousy of life and the joys they may no longer take part in, if they ever did in the first place, is the driving force behind all their actions.
According to some scholars the Wight-Fiends began as a race of Xenos, now long dead. How it came about is not known, but it was said that in the waning years of this race their world had become a form of Hell all it's own. Life had become nothing but suffering and sorrow, barely worth living if it was at all. In this environment of agony and fruitless survival a prophet was born. One blessed with the seed of power. This young Psyker looked upon his world and people and despaired. What was the point of living in such a state? Then he began to hear voices. Voices from beyond. Many attribute the voices to Nurgle, Maldizmus or some other malevolent presence within the Warp, but it can not be known for sure. These voices told the young Psyker that a release could be achieved, that he and his people did not have to continue in this state of endless agony, that there was salvation beyond the realms of flesh and blood.
So the prophet went amongst his people and preached of the true salvation that could be found beyond the doors of death. He was met with resistance, but when he found it he crushed it without remorse. Those left behind would merely suffer more after all. Eventually he united the whole of the race into his cult. This was not so difficult a thing for he had promised them a land of joy and peace in the other world. He had tought them that both life, and the world of the living were cruel things that wished them only suffering. A message which many had taken as truth, for their lives had shown them little to tell them different. Finally all was prepared.
The prophet prepared a ritual that he had learned by trafficking with the spirits of the beyond. It was a single grand sorcerous ritual that would allow his whole race to ascend to their paradise. When the time came the entire race died in ritual suicide, ready to abandon this world for a new home. The psychic backlash of the ritual scorched their home planet, rendering it a Dead World.
However while the race was now free from its mortal bonds and worries they had been deceived. Through the hardship they had suffered and by the dogma that their savior had given them they had been taught to hate life. Now as they cast themselves into the Warp their bitterness and spite towards life and the foul magics of the ritual twisted them, changing the shape of their very souls. They became monsters filled with hatred and jealousy towards life. There was no paradise awaiting them, only more hatred and an endless grave. These were said to be the first Wight-Fiends.
More Wight-Fiends would come into being later. Often they are the spirits of those that died via suicide or otherwise have some grudge against life as a result of the cruelties they suffered while living. Existing Wight-Fiends often seek these spirits out, whispering into their minds and drawing them back to the Endless Grave so that they may corrupt these souls further, turning them into new Wight-Fiends. Some are not even dead when they are recruited. Certain insane mortals have been known to summon Wight-Fiends in order to satisfy some grievance they have with their lives, or the world around them. Some even seek out these malevolent Warp Entities with the hopes of joining there ranks. To these mortal Wight Fiends will often teach a ritual, similar to the one that brought the original Wight Fiends into existence but of smaller scale. This ritual which culminates in suicide allows the mortal to transform into a new Wight-Fiend on death, granting them the power that they would have used to bring death to the loathsome world around them had they not lacked it previously.