Warhammer 40K: Who are the Thousand Sons?

The Warhammer 40,000 universe is full of heroic factions like the Ultramarines and terrifying alien threats like the Tyranids, but that’s only a fraction of the whole picture. As seen featured in Warhammer 40,000: Space Marine 2, the Thousand Sons are a group of Chaos-corrupted Space Marines with a penchant for magic and the occult. However, the Thousand Sons are more than just evil wizard marines. Their past is marked by tragedy and the fickle hands of fate, truly showing that the road to hell is paved with good intentions.

Undoubtedly, one of the most significant and influential forces in the Warhammer 40K universe is Chaos. It manifests in various forms, from daemons summoned through ritual to heretic Adeptus Mechanicus members creating diabolical homunculi of steel and flesh. However, the most iconic representation of Chaos is the heretic Astartes. Each warband can trace its lineage back to one of the traitor Legions from the Horus Heresy civil war, and most have willingly surrendered to the ruinous powers, seeking a share of their dark might.

The Thousand Sons were one of the Legions marked as traitors during the Horus Heresy, but it’s not as cut and dry as the other heretics. While there is no excusing the vicious actions of the group in the lore’s current standing, for quite a while, the Thousand Sons were acting in the best interest of the Imperium.

A tragic past

Like all Adeptus Astartes, the Thousand Sons started out as loyal members of the Imperium. Their Primarch, a demi-god genetically cloned from the Emperor, Magnus, grew up on the Psyker world of Prospero, where he sharpened his already potent psychic abilities. He soon surpassed all his teachers and set about advancing civilization on Prospero, making Warp study a primary focus.

Magnus was repeatedly warned about the perils of the Warp and the entities that dwelt within, both from his teachers and the Emperor after the two were reunited. Despite this, Magnus frequently projected his psyche into the immaterium to scour its depths. Following the Primarch and Emperor’s reunion, the Space Marines that would become known as the Thousand Sons were Magnus’ to command. Like him, each bore a natural gift for Psyker arts and a fascination with the Warp. However, his genes proved unstable when applied to his sons.

The Space Marines tended to become afflicted with something known as the “flesh change,” which would see them mutate into mindless abominations. While some called for the Legion’s disbandment and destruction, Magnus pleaded for the chance to save his sons. Eventually, after many attempts to plumb the Warp for a cure, Magnus seemingly found what he was looking for. The Primarch saved those he could, leaving only around a thousand of his Legion unaffected, earning the Space Marines the name the Thousand Sons.

The Council of Nikaea

While the Thousand Sons proved as effective as any other Astartes Legion, the other Primarchs and their sons looked on with dread. As the Imperium pushed further into the galaxy, it encountered vile alien species that used Warp magic, much like the Thousand Sons. Psychic abilities became taboo, and Magnus’ Legion was perceived as skirting too close to those ruinous powers. The issue culminated in what became known as the Council of Nikaea, where the Emperor called a meeting between all Primarchs and other trusted advisors. The righteousness of Psykers would be put on trial.

Though Magnus did have some voices in his corner, the events at Nikaea did not go in his favor. Except for Navigators and Astropaths, needed for space travel, the Emperor decreed that Psykers were not permitted to practice. Magnus begrudgingly accepted his father’s ruling, but it wouldn’t be long before he’d break his promise. The Primarch would foresee the Horus Heresy and all the ruin it would bring. Desperate to warn his father as quickly as possible, Magnus used astral projection to traverse the Warp and reach the Emperor directly.

Just before reaching Terra, Magnus found his path barred by some kind of seal. At this time, a voice reached out to him in the Warp, promising to help the Primarch overcome the barrier. Seeing no other option, Magnus accepted the help and crashed through the obstacle right into the Emperor’s throne room. The Emperor put the seal in place to keep daemonic forces from reaching Terra, and now they were invading through the hole Magnus had created. The events also destroyed the Emperor’s Webway project, which would have allowed humanity to discontinue its use of Warp travel.

Prospero falls, Chaos rises

When Magnus returned to his physical body, the mysterious voice revealed itself as Tzeentch, the Chaos god of change and sorcery. The entity told Magnus that he and his sons were destined to serve Chaos, one way or another. Meanwhile, the Emperor ordered the Space Wolves and their Primarch, Leman Russ, to bring Magnus and the Thousand Sons back to Terra to answer for their crimes. This order was intercepted by the now-corrupt Horus and was changed to see the Space Wolves decimate Prospero and the Thousand Sons.

Magnus fully realized that he had become a pawn to Chaos and opted for inaction. He believed that any further struggle would play right into the hands of Chaos, and he was ready to allow his Legion and himself to fall for his folly. However, after watching the destruction start to unfold and believing that he had been in the right trying to warn his father, he took to the field to deny his Legion’s execution. That ultimately led to a confrontation between brothers as Leman Russ and Magnus faced each other in battle.

Russ eventually overpowered Magnus and, in one swift blow, broke the sorcerer Primarch’s back over his knee. Completely without hope, Magnus heard the voice of Tzeentch again. It offered to save him and his remaining sons if he swore allegiance to the dark power. Seeing no alterative to save his sons, Magnus agreed. The Primarch and the Thousand Sons were transported to a world within the Eye of Terror known as the Planet of the Sorcerors.

The Rubric of Ahriman

After the events of the Horus Heresy played out, the Thousand Sons would be part of another Legion-defining event. Though the Thousand Sons were now sworn to Chaos, the Legion’s rampant mutations returned. Azhek Ahriman, Magnus’ trusted advisor, was unwavering in trying to find a cure. In secret, he formed a cabal of sorcerors hoping to unlock the secret to ridding the Legion of its mutations, but it ultimately backfired.

Ahriman completed a ritual that saw the “flesh change” cured, but it ravaged those in the Legion without a strong psychic connection. Sorcerors were unharmed, but everyone else was reduced to little more than dust sealed in power armor. The Space Marines were still alive in some sense, but only their souls remained under the armor to power the new automaton. Needless to say, Magnus was furious when he realized what Ahriman had done to his sons. 

Though the Primarch wanted to kill Ahriman, Tzeentch intervined and noted that the sorcerer was still valuable for furthering Chaos’ will. To that end, Magnus let Ahriman live but sentenced him and any of his collaborators to exile.

Sorcerous tactics

Being Chaos Space Marines with a history steeped in the occult, it should be no surprise that the Thousand Sons favor psychic abilities and magic. The heretics are just as comfortable summoning daemons to do their bidding as they use Warp-infused weaponry. That’s not to say that they don’t use similar gear and weaponry as other Astartes factions, but some kind of psychic might almost always supports them.

Like their patron Chaos god, the Thousand Sons enjoy twisting schemes and meticulous planning. That could come in the form of an attack planned at precisely the right time, as certain celestial bodies align, or a long game of corruption for unsuspecting Imperium citizens. The Legion’s goals can often be obscure or seemingly irrelevant until a later revelation announces true intentions.

While the Thousand Sons’ fall from grace was tragic, there is no redeeming the Legion now. It has committed itself to the dark forces of the Warp and as an enemy of the Imperium. And while the Thousand Sons carry out countless acts of heresy and damnation, Magnus still contemplates his hatred for the father he believes betrayed him.