Each of the Primarchs were created with different abilities that the God-Emperor of Mankind thought would be useful during the Great Crusade; the second was given the Blank gene. This ability did not prevent the second Primarch from being snatched up by Chaos and scattered to the desert world of Voralia, but his null-aura was sufficiently strong to frighten away predators and scavengers, allowing him to survive his first few nights alone.
One night, he chased some Sandlegs (mildly domesticatable wolf-sized arachnids) away from a carcass, and noticed that they fled in one direction. He followed them, and they led him to the hermit Maloris, who in turn took the young Primarch to the nearby village. The villagers had seen the second Primarch's genetor-pod falling to Voralia, and taken it as an omen. When they realized that the pod had borne a child from the sky, they immediately accepted him.
Maloris adopted him and named him Rachnus Rageous, and while his null-aura was off-putting, he grew quickly, and impressed his fellow villagers. He came to be known for his reliability as an ally and his ferocity as a warrior, and he became close friends with a few youths in his village: Fal, Ridons, and Caustan.
After Rachnus successfully completed his manhood trial by killing a massive Sandleg, the God-Emperor of Mankind appeared in a blinding flash of light, where he revealed the truth of Rachnus's origins. Rachnus bid a tearful farewell to the village that had taken him in, and took command of the Desert Fangs Space Marine Legion. His spirits were somewhat lifted when his friends became Desert Fangs as well.
Thanks to their gene-seed, all Desert Fangs possessed an innate resistance to the powers of Chaos, while Blanks' abilities were dramatically enhanced. As such, the Desert Fangs were extremely effective at liberating Chaos-held worlds.
The Desert Fangs became close allies with the Alpha Legion when they cooperated to liberate the world of Zeol. Alpharius (or Omegon) quickly realized the tactical potential of the Desert Fangs. First, he used the Legion's Silencers to mask strategic planning sessions from daemonic scrying, while loudly discussing false plans when the Silencers weren't around, in order to misdirect the enemy. He then suggested that the Desert Fangs use their Sandlegs to burrow directly into the main ritual sites, where a Silencer task force could decapitate the Chaotic occupiers in one strike. The plan worked flawlessly; Rachnus himself, along with his Sandleg Fire Strider, annihilated a Lord of Change in single-combat, and after the ritual site was destroyed, the resistance collapsed, and Zeol was held by the Imperium within a week.
The Desert Fangs cooperated extensively with the Alpha Legion after that, and were on good terms with the Space Wolves, with healthy (and distant) respect for the Thousand Sons, but their tactical unorthodoxy led to friction with Roboute Guilliman and the Ultramarines. This tension came to a head at the meeting at Ullanor, when Rachnus threw a gravyboat at Guilliman's head, starting a food fight among the Primarchs.
Horus was aware of the Desert Fangs' service record against Chaos, and perceived that they might be a threat to his rebellion, especially as nearly the entire Legion was part of the force attacking Isstvan V. To divert them, he used his position as Warmaster to influence Tialoc Ekans, Primarch of the Sky Serpents, to attack the Desert Fangs' homeworld. Alpharius warned Rachnus that his homeworld was in danger, and so the Desert Fangs fled to Voralia. Unfortunately, they were too late to stop the Sky Serpents' fleet; with the Sky Serpents between them and their homeworld, the Desert Fangs fled back to the Isstvan system, hoping to draw the Sky Serpents away from Voralia, but their gambit failed (though this did give the Desert Fangs some time to retrieve the remnants of the loyal legions). When the Sky Serpents did engage the Desert Fangs, the latter were so enraged over the loss of their homeworld that the Sky Serpents were all but annihilated. Fal was mortally wounded in the attack, but his rage was so strong that he lived long enough to be entombed in a Dreadnought; in fact, such was his rage that he would later be recognized as the first Belligerent Engine.
Rachnus was contacted by Alpharius while his Legion regrouped from Tialoc's attack; Alpharius briefly explained that the Chaos Gods were very real, that Horus had been corrupted by them, and that the Alpha Legion was hoping to destroy them from within. He urged Rachnus to go to Terra as quickly as possible.
After licking their wounds and regrouping, the Desert Fangs (and assorted rescued loyalists) raced to Terra, arriving on what would be the final day of the siege. Rachnus, a redemption-seeking Tialoc, and a force of Silencers entered the Vengeful Spirit in a stealth ship (aided by their null-auras), and managed to fight their way to the bridge. Rachnus was too late to stop Horus from mortally wounding the Emperor, but he plowed through Horus's bodyguards and engaged him at close quarters, disrupting Horus's control over his powers enough for the Emperor to return the favor.
The God-Emperor of Mankind recognized, as Alpharius had, that the Desert Fangs were his most potent weapon against Chaos, and that it was only a matter of time before the Ruinous Powers wised up to their existence and started a Black Crusade against them, against which they would surely fall. To preserve their secret, he had all records of the Desert Fangs and their deeds destroyed, and ordered Rachnus and his Silencers to go into hiding. Fal took control of the remainder of the Desert Fangs, whom he then renamed the Angry Marines and recolored in red and yellow. To complete the historical erasure, the newly-minted Angry Marines modified their gene-seed so that it would no longer grant them a mental ward against chaotic influence (although their newfound rage at the loss of their history more than replaced it).
Having been removed from the galactic radar, Rachnus and his Silencers were left to pursue their crusade against Chaos in secret. He quickly established a base on a world (named Maloris, after Rachnus's adoptive father) in the outskirts of the Eye of Terror in order to grow a population of Blanks, so as to replenish losses. His most recent strike against Chaos was an attack on a Slaaneshi Daemon World, where he confronted Fulgrim; unfortunately, the daemon controlling Fulgrim's body escaped, but he was severely injured in the process, and Rachnus's fleet was able to destroy the world and strike a devastating blow against the Emperor's Children.
What other adventures Rachnus had afterwards is unknown, but his crusade against Chaos is far from complete.
While the Angry Marines were pretty much always envisioned as one of the lost chapters, /tg/ had originally envisioned Rachnus much differently. Instead of landing upon some planet, Rachnus' pod hit a human spaceship. Rachnus was so pissed at the Chaos gods that he murdered everyone on-board using one of his victims as his weapon. This version of Rachnus' origins was short, sweet, and to-the-point. Perhaps too much so. Thus, when the Angry Marines were shifted over to their "toned down to make it work" version known as the Desert Fangs, the story of Rachnus himself was given the same treatment.
There are also two primarch stories on the Angry Marines page, one about "Asura" and one similar to the above but involving Battletoads. The note "possibly Guts" is also in the infobox. There is also a Warhammer High entry about "Enfuratori" (or "Enfuratus", it is that kind of fic).
- Desert Fangs; Rachnus was Primarch of this Legion.
- Angry Marines; the present identity of the Desert Fangs.
- Knights Inductor; Reasonable descendents of the Desert Fangs/Angry Marines, made by a rogue Magos Biologis without their permission.
- Lost Primarch Quest; a quest in which /tg/ determined what actions Rachnus should take.