Well...I did something similar...
See this:
http://www.heresy-online.net/forums/showthread.php?t=2430
This is my take on the History of the XV Legion:
Magnus was the sorcerer supreme among the Primarchs, and his chief Librarian was Ahriman.
The XV Chapter had a high percentage of psykers in their midsts, but not all of them were psykers. Think of having two maybe 3 times the normal number of psykers...which is still rare. Say maybe half a dozen in an average chapter and a chapter comprises the very best of the best that survive the whole astartes genetic program.
The Thousand Sons refer to the original 1000 Astartes made from Magnus' Geneseed. From that they created the XV legion "The Thousand Sons". They were still largely astartes like any other: warriors, soldiers, elite, but Magnus became more and more distant as his studies took him deeper in to the arcane.
Now at this point, nobody is wosrhipping anyone or anything. Loyal servants of the Emperor, beyond the heresies of gods and all that. They simply seek knowledge and protect themselves in ritual: symbols, signs, runes and other accoutrements of psyker power. Over time he was able to build his power as did Ahriman and his elite cabal of Librarians who became more and more in charge of the Legion and Ahriman became the de-facto leader of the Legion at the end as Magnus was simply too busy with his own studies.
Now with the heresy upcoming, Magnus became aware of Horus' plans, and tried to warn his father...the Emperor. Now of course, he had been warned about his use of sorcery (thanks to the council of Nikea, and their fears about the use and abuse of psyker power and the entire point of the great crusade and the Emperor's primary drive: to wipe out the last vestiges of religion and mysticism which had held humanity back for millenia) but with the severity of the situation, Magnus tried anyhow to inform the Emperor.
This backfired, trusting Horus above all others, his #1 favored son over the son who simply wouldn't obey...The Emperor sent Leman Russ and his Space Wolves to bring Magnus to heel. Not to slaughter them, but to bring Magnus to justice. Of course, Magnus and Russ had been bitter enemies from the start...and Russ came loaded for bear.
Magnus wouldn't be taken, and his response was arcane. The defense of Prospero was pathetic. Too many Astartes unprepared due to their Librarian leadership being too deep in study, or otherwise too busy to prepare a defense. The result was a slaughter of Thousand Sons. Magnus actually was able to remove Prospero to safety within the Eye of Terror.
After this The XV Legion had no choice but to join Horus. The slope was slippery and soon simple psyker power usage and ritual usage became soon worship of Tzeentch in order to survive the coming storm.
After a long time, centuries, from being in the Eye of Terror and having a high numbers of psykers in the legion who regularly opened themselves to the emperian...mutations started to show. A few started early, but the progress was exponential.
Again, Magnus was distant, busy with larger more galactic-level issues while Ahriman was left to run the legion. The Thousand Sons were falling apart due to mutation before their very eyes. Ahriman researched and enacted a powerful ritual that would forver change the legion and bear his name: the Rubric of Ahriman.
The result was that the astartes of the legion with little to no psychic potential were instantly and forever sealed in their armor. These all turned to dust within a century and the legion has changed it's look since as well, taking on the blue and gold tomb-like sarcophagi look to resemble the dead that they really are.
Of course, Magnus was enraged when he found out about the Rubric and how Ahriman had essentially destroyed his legion. Just at the moment when he was going to break Ahriman in half, he was paused by the laughing voice of Tzeentch who indicated that this was all a part of his greater plan. Ahriman was spared, but banished from the legion.
