I think I should be fine with the Alpha Legion, though I'll know for sure after a week or two's work on them. Now onto some Viciously Nitpicking Sig Feedback 
Magnus was the first of the Primarchs to recognise the presence [would 'existence' work better?]
and the threat the Ruinous Powers posed to Mankind, and because of this became the first legion to be targeted for destruction by the forces of Chaos. Though it cost them their homeworld, the few survivors [perhaps 'the survivors of the Space Wolves slaughter' would add a bit more detail to this sentence]
were still able to play a pivotal part in bringing the Dornian Heresy to an end. As the most powerful battle-psykers in the Imperium they are feared and mistrusted even by the people they selflessly defend against the encroaching darkness.During the great diaspora, when humanity colonised the galaxy, the world of Prospero was chosen precisely because of its remoteness and lack of mineral resources as a haven by the psykers and mutants who had settled there. [Sentence seems a bit unwieldy to me, could be better organized. A sentence before it about the stigma about being a psyker might not be out of place either]
In their splendid isolation they created Tizca, the shining city of light, learning and culture amidst the barren wasteland. Although they had been cut off for thousands of years, tales still abounded of the howling mobs that had persecuted their forefathers. They lived in quiet dread that their haven would be invaded, so when the infant Magnus’s incubation chamber fell to earth there, the population was gripped with terror. [This sentence could be better organized as well, so it has a bit more of an impact]
The vessel landed amongst the shining silver towers in Tizca’s central plaza. Finally overcoming their trepidation, they found it to be not an orbital bombardment or the vanguard of an invasion force, but a battered, scorched and damaged wreck containing a badly injured child.
Though he was near the point of death when they first found him, the boy displayed a tenacious will and remarkable constitution. Under the ministrations of the commune’s healers he quickly returned to robust health, but nothing could be done to save the boy’s right eye. An ugly welt of scar tissue covered the socket, but in a society where mutation and deformity were rife, this was little cause for comment. [Could this be absorbed into the previous paragraph? It would make the article neater to my mind this way]
The boy was named Magnus, or, more affectionately, Magnus the Red after his ruddy complexion and the colour of the unruly mane of hair that he doggedly refused to cut. He quickly grew to vibrant maturity, a towering giant of a man, but nevertheless threw himself wholeheartedly into learning. Magnus was most often to be found poring over the ancient tomes in the city’s extensive libraries and animatedly debating with frail and [in interests of keeping the word count down, this could be cut]
wizened scholars many times his own age. Knowledge was his passion, and with the fiery certainty of youth he believed that no subject was beyond his mastery. It was the study of the hermetic arts of alchemy and sorcery that changed his life forever [perhaps the wording is a little cliche? How about 'altered the path of his destiny' or along the lines of that]
. In doing so he opened his eye [would 'his mind's eye' work better?]
to the true power of the Warp, an act which levelled the building in which he had been studying. As his psychic powers blossomed, he became aware of both the Gods of Chaos, and his father, The Emperor of all Mankind.
The creature’s razor-sharp claws shredded old Master Colpek without breaking its stride, and advanced on Magnus. They had summoned a creature of nightmare... His nightmares, in fact [the 'in fact' sounds out of place]
. He recognised the same rasping growl and the way its talons moved like liquid flame; it was the thing from his earliest memories that had torn its way into the vessel that had brought him to Prospero. Then it had taken his eye. Now it was back [would 'returned' work better here?]
Something had happened to him. At first his skull felt as though it was splitting open, but now Magnus felt as though his mind had opened to something far beyond the material plane. His awareness snapped back into the hermetic tower to find himself alone with the entity... the daemon. Before he could react the thing was upon him, a talon pinking the flesh around his one good eye and whispering to him in its sharp, guttural speech. It was here to stop him from becoming a powerful tool of his master’s rival, it said. The names it spoke of, ‘Khorne’ and ‘Tzeentch’, resonated through his soul like shards of ice.
Despite his desperate struggles the creature whispered that he would first take his eye to complete the set, and then pluck out Magnus’s heart to appease his master. Then, just as the talon broke the skin beneath his eye, calmness descended. Using previously untapped potential [sounds a bit out of place to me]
he reached out with his mind and snuffed out the daemon’s very essence. The ensuing explosion could be heard all across Tizca. Untouched by the conflagration, Magnus considered the daemon’s words. He would be no puppet of this Tzeentch, and no one would take his eye.
Though separated by half a galaxy, The Emperor and Magnus recognised each other for what they were. With his insatiable thirst for knowledge, Magnus’s eagerly learned about every aspect of the burgeoning Imperium, of the Great Crusade, and of the search for his brother primarchs. Through his expanded acuity, and the tutelage of his father, Magnus was the first of the primarchs to learn the terrible threat posed by the Ruinous Powers. Even as The Emperor taught him these things, He swore Magnus to secrecy about them, explaining that the threat was so grave that even knowledge of their existence would be enough to seduce some to their worship. Magnus had seen to his cost the palpable link between sorcery and Chaos, and argued that ignorance of the Ruinous Powers could as easily be used to corrupt the unwary, but was unable to persuade his father. The Emperor spoke of a grand plan that would one day see humanity strong enough to resist Chaos’s wiles
, but until then, He forbade Magnus from telling anyone outside his legion, even his own brother primarchs, of what he had learned.
By the time The Emperor and the Fifteenth Legion of Astartes had reached Prospero, father and son already knew each other as though they had never been parted. Despite all of Magnus’s assurances, the arrival of the Master of Mankind and His fleet of warships were greeted with suspicion by the people of Prospero, but The Emperor’s legendary charisma quickly won them over. To further assuage their fears, Magnus had his legion build their Fortress-Monastery at the heart of Tizca in the hope that they would no longer have to apprehensively watch the skies for signs of attack.
The legion itself had particular reason to celebrate finding their primarch. Although they had not recruited on the basis of psychic talent, the gene-seed they had been patterned upon had boosted their innate abilities and triggered latent ones. As a side-effect it also caused a very high incidence of both genetic and implant mutation among the brethren, to the extent that the legion was smaller than most others by the time it was united with Magnus. As the home of the mutant and the psyker, Prospero was the perfect place for such a legion, and they embraced their gifts wholeheartedly. By the time Magnus was ready to join the Great Crusade, his original ‘Thousand Sons’ had expanded moderately beyond their nominal level by the cream of Prospero’s talents. As they set out to do The Emperor's great work, what they lacked in conventional numbers was more than made up for with highly trained battle-psykers able to kill an enemy with but a thought. WIP
, more to come...