This article or section is EXTRA heretical. Prepare to be purged.
Magnus the Red is the primarch of the Thousand Sons, and rules over the Planet of the Sorcerers. He was
sort of a badass once. He is notable for having a big ol' red eye right in the middle of his forehead, red skin and being hueg liek exbawks, even as far as SPESS MEHREENS go. Depending on who you ask, he may also be totally is the Primarch of the Blood Ravens, which would explain both their color scheme, high percentage of psykers, and Hair-esy.
 The Great Crusade
Magnus landed on the planet of Prospero (A planet whose ancient civilization was composed predominantly of psykers, who had fled there because, by and large, Psykers were persecuted by humanity writ large). As Magnus was himself a Psyker, he formed a kinship with the people of Prospero in short order. Taken in by Prospero's people, he absorbed knowledge like a sponge, and became crazy powerful, and in time far exceeded the power and control of all of his teachers. In time, he had become the single most powerful thing on the fucking planet, and he led a campaign to eradicate a race of predators on Prospero known as the Psychneuein, which overran many of Prospero's early cities and had a fondness for laying their eggs in unsuspecting psykers' brains. The beasts had destroyed the entire population of Prospero, save for its capital city, Tizca. One by one, bit by bit, Magnus' forces retook the planet, putting the Psychneuein to the sword, and within a year, Magnus had completely reclaimed the entire world (except for the Desolation of Prospero).
Once the Psychneuein were destroyed, Magnus became the planetary leader by popular demand. Magnus decided to be fucking awesome, and he rebuilt the cities. Like a somehow perfectly stable fortress some 50 years in Dwarf Fortress, Tizca became arguably the most beautiful in Imperial space, with crystalline spires and pyramids, long marble roads, and psychically-resonant crystals in key locations, designed to calm the turbulent minds of younger psykers and help them control their burgeoning psionic potential easier. The city quickly became known as a shining jewel of humanity, and one that showed proudly how far its citizens had come from the brink of near-extinction. Prospero also had one of the most technically advanced defensive networks in the Imperium, all of which was hilariously wasted except the shields when Magnus in a fit of Primarch-scale angst over his "I screwed everything Dad was doing and failed my own purpose entirely in the process" whoopsy-daisy, decided that the Space Wolves should get the rabbit punch.
All this time, Magnus continued to codify and systemize everything he could about the 'Great Ocean' - what the Prosperans called the Warp. Huge libraries filled with knowledge about psychic powers and the warp were established, and Magnus himself used his powers to peer into the Empyrean itself, claiming many of its secrets, though at terrible risk to himself. While this was exceptionally dangerous, much of what the Imperium's Inquisition currently knows about the Warp came directly from several of his manuscripts that survived.
With such a powerful mind heading into the warp, it was inevitable that the Emprah would eventually take notice. So he finally came to Prospero, and he and Magnus psychically brofisted before talking for several days. The Emperor taught him even more about psychic powers, but cautioned Magnus to slow down and be careful, since foul horrors lurked in the warp and liked nothing better than forcibly sodomizing an unprepared Psyker's soul. Magnus took much of this to heart - but what the Emperor did not know was why - Magnus' forays into the warp had already came at a price - his right eye, sacrificed ala Odin when he had consorted with warp-entities whose nature he did not understand until the Emperor explained shit.
Magnus was put in charge of the Thousand Sons, but would not immediately embark on the Great Crusade - so many of his Legion were psykers, and there had been so many small mutations that a number of them couldn't survive the gene-seeding process. Even worse, during combat psykers ran the risk of losing control of their psychic powers and undergoing the "Flesh Change", where their body mutated rampantly out of control at all the Warp energy running through them (we call them something else, but won't speak its name as...wait, I didn't say it's naSHHTLSUROHSONTOOLS!) Magnus had been aware of the mutations his people were suffering, and he had entered the warp previously to gain answers as to the cause and cure - though Magnus didn't know it, he had consorted with Tzeentch right before he arrived, and the god of Just as planned saw a lot of potential in this one.
In any case, because of the setbacks that had befallen the Thousand Sons, the selfless efforts of Magnus to save them, their shared psychic talents, and the continuing suspicion of many in the Imperium and among the other Space Marine Legions towards the Thousand Sons' use of psychic abilities and the rampant mutations present in their gene-seed, the Primarch and his Astartes developed an extremely close emotional and psychic bond - arguably the strongest such bond among all Primarchs and their Space Marine Legions, exceeding even the Luna Wolves' dedication to Horus and the Blood Angels' dedication to Sanguinius.
Finally, about midway into the Great Crusade, 100 Terran years after it had begun, the now "stable" and full-strength Thousand Sons Legion, with Magnus leading them into the field, were permitted to take part in the effort, as the 28th Expeditionary Fleet of the Great Crusade. And it was glorious. When they were called to the field, the Sons were a sight to behold, flinging forth psychic storms to consume enemies of the Emperor and striking with both precision and prescience, using their abilities to ascertain enemies' weak-points. Magnus was unique among the Primarchs in that he would always try diplomatic routes first, however - in this way, the Thousand Sons won many battles without so much as a shot fired. This did not sit well with Rogal Dorn or Guilliman, who viewed this as unmanly. Likewise, Magnus drew constant distrust from Mortarion and Leman Russ, who distrusted the Sons' sorcerous ways and were somewhat concerned by a trait the Sons had picked up: after conquering or annexing a world, they would take huge amounts of knowledge - books, scrolls with forgotten lore, and ancient artifacts - back to Prospero for study, analysis, and codification. And this doesn't sound like the Blood Ravens, some say? Nicking everything shiny and not nailed down was the Thousand Sons thing, and nobody looted so prolifically.
Though when diplomacy didn't work Thousand Sons was known to fuck entire armies with lightnings, fireballs, mass mind control and precision strikes dealing so much devastation with only few marines, that entire companies of Iron Warriors would whip in tears of jealousy, if they ever cross Sons during Great Crusade. Magnus himself once fought Ork gargants by himself, and despite being able to zzap, melt or transmute their hulls with just power of his mind he instead scaled himself to the size of Warlord class titan, and beat the shit out of Gargants with his bare hands.
It wasn't long before Magnus was called to task by his brothers. The Sons' obsession with preserving valuable knowledge - whatever the source - pissed off several of the other Primarchs. The Thousand Sons' political opponents among the other Astartes Legions found their methods useless, and arguably counter-productive to the goals of the Great Crusade which sought to spread the specifically Imperial strain of human civilization across the galaxy, and possibly dangerous to boot, since the Warp was so dangerous. Arguments between Magnus and the other Primarchs became so pitched and so violent that on one occasion, Leman Russ and Magnus got into a giant fucking brawl on the council floor, and it only didn't end in bloodshed because Lorgar managed to talk both of them down, causing Russ to grudgingly holster his weapon and Magnus to stop chargin' his lazar. Finally the Emperor stepped in, bitch-slapped everyone for acting like a retard, and pointed out that whilst dangerous, Psychic powers were extremely necessary to the Imperium writ large. Chaplains were added to Space Marine chapters in order to ensure adherence to the Imperial creed, and Psykers were to be properly sanctioned by the Imperium. Magnus was ticked off, but calmed down a bit a little later and continued the Crusade with the new restrictions in place (actually, they carried on with their arcane ways in secret. Read A Thousand Sons).
 Horus Heresy
When Magnus learned that Horus intended to betray the Emperor and slaughter the other Astartes Legions, he resolved to use any method possible to warn the Emperor. With no other option, he used his sorcerous powers to open an Eldar Webway portal and used it to travel within range of the Emperor. Unfortunately, his doing so destroyed a top-secret Webway project that the Emperor had been working on for months, which would have rendered dangerous warp-travel obsolete and would connect the Imperium of man using a human-engineered version of the Eldar's webway gates. He called Magnus to task for using Sorcery to activate the webway on top of this, refused to listen to Magnus about Horus, and declared Magnus a DOUBLE HERETIC. Magnus fled the scene rather than fight with his own father, realizing that the Imperium was comprehensively fucked and that he needed to do all he could to protect it and Prospero.
He doesn't get much press-time because of how bad he and his Legion got dicked by Tzeentch - but when he gives enough of a shit to get up and join the fray, you watch your ass.
When Horus learned of Magnus' attempt, he was quietly amused. The Emperor ordered Horus to have Russ bring Magnus back to Terra to stand trial for what he had done, and Horus, being a dick, quietly altered Russ' orders to lay waste to Prospero and slaughter the Thousand Sons. As Magnus was already a declared double heretic and he hated Magnus even before this, Russ took to this like a Furry to Ironclaw. Accompanying the Space Wolves were a full contingent of Adeptus Custodes, millions of Imperial Army troops, and the elite Imperial anti-psyker unit, the Sisters of Silence (think a unit full of Culexus Assassins and you get the general idea).
Magnus, sensing this, and realizing that this had been Just as planned and that Tzeentch had done this intending to completely destroy both the Thousand Sons and the Space Wolves decided to dick with the Lord of all Fate: He decided to accept the destruction of everything that he had worked for, so that Tzeentch's ultimate goal would only be half-fulfilled. Tzeentch was richly amused by this. As Prospero burned, Tzeentch and Magnus engaged in act after act of dickery and counter-dickery, with Magnus finally pushed into a towering rage and taking to the battlefield at his capital, crushing his enemies with volleys of MIND BULLETS before he engaged Leman Russ in close-combat. The two fought fiercely, when Magnus managed to Falcon pawnch Russ so hard it shattered his breastplate and punctured one of his hearts. Russ was still in the battle (mostly because he had back up in the form of two fucking massive wolves) and after getting lucky and taking out Magnus' awesome eye, performed a back breaker on him and ended the fight. Tzeentch was greatly amused by all how this panned out - it's rare that anyone, let alone a mortal man, could try to out-manipulate the architect of fate himself. He thus made Magnus an offer: Become Tzeentch's servant, and preserve what was left of his legion and homeworld, or watch what was left of both burn as he lay dying.
...For Magnus, it never really was a choice. The response of Magnus' new patron was immediate - for once, Tzeentch was true to his word. The City of Light was transported into the Eye of Terror to a new Daemon World. Prospero was destroyed that day, but Magnus and his Legion survived. By the time the Thousand Sons were seen next, they had joined up with Horus' force of Traitor Legions on their way to lay siege to Terra (not actually confirmed), and Magnus the Red had become the most powerful of all Tzeentch's daemonic servants. Magnus had never served Tzeentch willingly, but now had no choice - exactly as Tzeentch had planned from the very beginning.
And so, the Horus Heresy came and went. The Siege of Terra occurred, Horus and the Emperor fought and (SPOILER WARNING) failed, and the Traitor Legions were driven to the Eye of Terror. However, the Thousand Sons now had to deal with more immediate problems: with their serving of Tzeentch, the Flesh Change returned with a vengeance. Magnus made efforts to stop this, but being a servant of the God of Mutation has its drawbacks. Growing desperate, Ahzek Ahriman, the Chief Librarian and First Captain, took matters into his own hands. Having lost his brother to the Flesh Change before they found Prospero, Ahriman gathered a cabal of other sorcerers, stole the Book of Magnus, and performed the Rubric of Ahriman in an attempt to stop the Flesh Change. The results were not what Ahriman expected: while it stopped the Flesh Change and further empowered all psychic Thousand Sons, all non-psyker Thousand Sons had their bodies turned to dust and sealed within their Power Armour, becoming little more than robots needing guidance from a Thousand Son sorcerer.
Magnus was furious. Confronting Ahriman, once his most favored son, Magnus was about to kill Ahriman when Tzeentch spoke to him: "Magnus, why do you seek to kill my pawn?" Once again, Magnus had been used. Broken, Magnus simply exiled Ahriman from the Planet of Sorcerers. For the last ten thousand years, Magnus has labored to restore his Sons bodies to their original forms, failing all the while.
He later showed up on FENRIS ITSELF and became the second Daemon Primarch after Angron to get shit done, rampaging through imperial lines and laying waste to everything in his path with MIND BULLETS and RAW PHYSICAL POWER until the Space Wolves responded and after a extremely hard fight
managed to banish him back Magnus phased out himself and teleported all his marines out of the Fang, after reaching his goal, sabotaging an experimental Space Wolf geneseed mutation cure, and killing an entire Great Company HQ, Great Wolf and almost killing Bjorn himself in the process as a nice bonus. To be fair, Space Wolves almost managed to actually banish him back to Warp, and Magnus was almost out of energy and barely able to sustain physical form at the end of battle because he had dementia from his time in warp. Khorne was so impressed by this badassery that he decided to give a little leeway to Khornate sorcerers.
Funny how two out of two Daemon Primarchs to have gotten shit done ended up being repelled by the actions of the Space Wolves. And the best thing? Unlike Draigo "beat Mortarion and the Death Guard silly singlehandedly" faggotry, this fluff is actually somewhat well written, with the Space Wolves being pushed to their very limits trying to repel the chosen of Tzeentch.