Well, I've finally defined the origins of the Legion and their primarch, Virgultur, the Harvester, Sire of the II Legion; and made a more defined lore, this is what I came up with:
Origins: Virgultur and the Undead Legion
The II Astartes Legion, the Sentenced, were doomed since their creation. Their Gene-Seed had a terrible flaw that, instead of prolonging their lifespan indefinitely, shortened it to less than a couple years, as their vital organs eventually stopped working. Because of this, they were the legion with the less recruits, as everyone knew that forming part of the II was a sentence of death, but the few members that there were had a grim determination, accepting their fate a not caring about their life in battle, knowing that one way or another, they were going to die. Because of this, the legion was forced to use their trainees in battle before they were implanted with the gene-seed, as they needed them to be fully capable when it happened, as after they wouldn't have much more time to learn.
Even with this extreme measures the legion was smaller every day, as the recruits were few and the astartes died quickly, preventing them from having any veterans.
It was just when the Emperor was about to end the legion that they found the death world of Messis, a place in where humans had prevailed in large cities ruled by the Necrophants, powerful psykers that used their powers to raise the corpses of the dead and bind them to their will, creating huge undead armies to defend the people from the many dangers of the planet, mainly the xenos creatures know as the “nightmare stalkers”, dark and slim creatures with many legs and claws sharp as swords that imbued their victims in fear just by looking at them with their bright green eyes.
That was before the Harvester arrived. They say that one day he arrived from the black skies, a child with the skin white as the snow and with eyes dark as the night. He was found near the city of Mausulon, by a farmer, Ghortan, who took him into his care to save the son his wife was about to have, for he would be his second son and in Messis everyone had to give their second sons to the Necrophants when they reached adulthood to be killed and risen as undead soldiers to defend the people. This children that were born to die were called “Virgultum”, the “branch that is cut green”, so Ghortan named his new son Virgultur, for a true name will only make himself like more the son he had just adopted. But Virgultur was not a normal human, and soon the gifts that he had started showing themselves. It was less than a month before he had to be given to join the armies of the dead that he presented himself to the Necrophants saying that he was willing to serve alive. At first they told him that that was not possible, for humans panicked when they saw the Nightmare Stalkers, so Virgultur told them that he would bring them the head of a dozen Stalkers if they would admit him in their army afterwards. Laughing, the Necrophants told him that they would not only admit him but that they would make him their general if he, alone, killed a dozen Nightmare Stalkers and brought them their severed heads. Accepting the challenge and forcing them to oath that they would do as they had said if he made it, Virgultur picked the scythe that his father used in the farm and went to the wilderness from where he had came. During the course of two whole months he hunted a dozen Nightmare Stalkers, severing their heads with his scythe, looking directly at their eyes but never being afraid. When he presented himself to the Necrophants they couldn't believe him, and some of them said that he couldn't be the general of their undead armies, as he was barely an adult, even though he was already taller than them all. Accusing them of liars and oathbreakers Vilgurtur killed them with the same scythe that he had used to sever the head of the stalkers, but broke it in the process because of the brutality with which he used it. The remaining Necrophants pleaded for their lives and said that the will honour their oath, so Virgultur spared them and took his place at the head of his dead soldiers. For two years he battled the Stalkers, not limiting to defending the cities, but hunting them into the marshes and damps from where they came, winning every battle because of his cunning and might. But he saw that there were too many monsters in Messis to be fought with the army he had, so he told the Necrophants to rise the Nightmare Stalkers they killed so that they would join their ranks to fight their alive brethren. Horrified by the idea they all excused themselves saying that they were not powerful enough to do it and saying that it was madness. But Virgultur was not going to give in so easily, so he told them that if they teached him how, he would do it himself. Again underestimating him, the Necrophants agreed, thinking that he would destroy himself in the process. But Virgultur learned eagerly, and one day, after a bloody battle in which a lot of Stalkers died, he made the dark rituals that the Necrophants had teached him and, even though it left him exhausted, dozens of Nightmare Stalkers rose to fight for him, their souls bound to his will and their bodies animated by the powers of undeath.
Terrified by what they had done, the Necrophants started plotting how to kill Virgultur, for he was getting too powerful. But the servants of the dark temples of Messis in where the Necrophants lived learned of this plan and, part for vengeance, part because they wanted Virgultur to free them from the Stalkers, they alarmed him, and in his wrath he killed the remaining Necrophants. Now rid of their protests, Vilgurtur started rising Nightmare Stalkers and other monsters to join his army, conquering much of Messis with their might, making it secure for humans to live and, because his new source of soldiers, he abolished the law of giving second sons to be killed.
But there was a region of Messis that he wasn't able to purge, a place called Pandemoni, marshes so infested of Nightmare Stalkers and other dangers that not even his armies could defeat them.
So he started animating much more humans and monsters, spending whole days in the sacred temples creating a massive army.
But all this power wasn't unnoticed. Very far away, in another place of the galaxy, the Emperor of Mankind noticed the psychic emanations that were coming from Messis, and quickly recognized the power of one of his sons. At the head of the II and XVI legions, the second of which had already reunited with their primarch, Horus, he went to that unexplored region of the galaxy with the hope of finding one of his lost sons.
Virgultur had created an army of hundreds of thousands undead humans and monsters and decided to march towards the final battle against the Nightmare Stalkers so, even though he was exhausted by the effort he had made and sometimes could barely stand on his feet he, at the head of his huge army, marched into Pandemoni. He and his undead warriors went farther than ever before, but the Stalkers kept coming and the undead army started diminishing. Even with their numbers being reduced by the thousands, they kept going on, as the dead feel no fear and, at last, they reached the centre of the marsh. There they found the Nightmare Mother, a giant monster that spawned the Stalkers and that, taking advantage of Virgultur’s weakness, took control over the undead Nightmare Stalkers. Totally in disadvantage as his forces turned against him, Virgultur decided that he would rather die than flee, so he fought monsters and his own undead alike until he was the only human standing. When his last strength was just about to leave him a bright light shimmered in front of him and suddenly a group of humans clad in a towering black armour were fighting beside him. At the head of them he saw a giant with a claw and a mace, taller even than him, which fought with a magnificence he had never seen before. With axes of thunder and weapons that spitted fire the newcomers soon made a circle around him and, without really believing it, he saw that they were winning the battle so, with a expression of relief, he fainted.
That was how Horus and his Luna Wolves saved Virgultur, Primarch of the II. After a long talk with his true father, the Emperor, Vilgurtur learned about the Great Crusade, and decided to commit himself zealously to the cause, as he had dedicated all his life to the protection of humanity. But to do that he needed a Legion, and his sons were dying. At the same time the Emperor reprimed him for using his powerful psychic abilities to such evil purposes, as he had not only risen the dead, but he had bound them to his will and had practiced his dark arts with xenos beasts, which had turned against him when he needed them the most. To avoid this happening again, the Emperor trained Virgultur personally in the art of the Warp, teaching him how to properly use his powers. When this training ended and Virgultur had mastered his psychic abilities, he took command of his Legion, but was horrified to see how fast they were dying and clearly saw that if nothing was done the Legion would disappear. Not wanting to lose his newly found sons he isolated himself in his personal chambers to think of a solution. He soon saw what he had to do. For two weeks he worked, mixing the rituals that the Necrophants had taught him with the knowledge the Emperor had given him. He would bind the soul of his warriors to themselves, making them stand and fight even though their bodies dyed, turning them into undead but keeping their free will. Afraid that his father wouldn't approve he decided to make the ritual as soon as possible and, with the approval of his Legion, it was done. The legionaries now where slower, as they hadn't as much control over their body, but they were tougher and stronger, as the power of the warp flowed through them to keep them “alive”. But eventually the Emperor found out and his wrath was terrible, almost dismissing the II Legion, no more called the Sentenced but renamed as the “Harvesters of Souls”, as he said that now they were tainted by terrible powers. But Horus himself, who had befriended Virgultum since he saved him in Messis, interceded in his favour, saying that the warriors of the II remained loyal, were stronger than ever and that they couldn't afford to lose a whole Legion in the midst of the Great Crusade. Not really convinced about it the Emperor gave in, but assigned regiments of the Legio Custodes to closely guard the Harvesters of Souls, and prohibited them from having a librarius, saying that the taint was too strong to let them access the warp. Ashamed about having displeased his father, Virgultur was eager to show the value of his legion, and soon marched to battle, conquering world after world to make the Emperor proud.
This was how the Harvesters of Souls were born.
This was how the Harvesters of Souls started to die.
So what do you think about it? It makes it easy to later on make the Legion fall, as they're already tainted by the warp, and gives me the undead lore I wanted to work with. Any opinions are welcome, the only name I'm set on is Virgultur (Virgultum being the ethimological origin of the word "verdugo", which means executioner in Spanish) and not particularly satisfied with "Nightamare Stalkers".
Still progressing with the Hands of Doom cloaks, so far I've only done another one, so it may take longer than I thought at first.
They rose from the dead to slay the living:
The Harvesters of Souls
, the II Legion and their various allies.