Thunder Warriors
The Thunder Warriors were the first soldiers of the man who eventually became the Emperor of Mankind. In many ways, they were the precursors to the Space Marines.
Origins
They were formed toward the end of the Age of Strife, when the Emperor judged that it was the right time to make his presence known. He needed some advantage over the warlords, barbarians, and other would-be world rulers, and so he created twenty regiments consisting of hundreds of genetically enhanced soldiers wearing simple powered armor. Because the Emperor's symbol at the time was a thunderbolt, and this symbol was prominently displayed on the chestplate of the armor, the armor came to be known as Thunder Armor and the warriors who wore said armor were called Thunder Warriors.
The Thunder Warriors were devastatingly effective and quickly became icons of the Emperor's armies; just the threat of the Thunder Warriors' arrival could convince a warlord to surrender, and those who refused quickly learned that the Thunder Warriors' reputation was not exaggerated. According to the fluff, they were even more physically powerful and ruthless than the Space Marines. But NOT more so than the Custodes. However, this said, according to the novella The Last Church, a massed Thunder Warrior formation could shoot all at once, and create a noise 'like a thunderstorm had suddenly sprung into existence', and mow down four or five men at once in that opening salvo. So yeah, make of that what you will.
And yet, for all their successes, the Thunder Warriors were not perfect; because the Emperor was busy running the Unification Wars, fending off attacks from his neighboring warlords, and lacking the resources and expertise of Mars, he and the few scientists he had working for him were forced to cut corners, and so the Thunder Warriors were all unstable in one way or another. None of them could be counted on to last long, as either their minds or bodies gave out at unpredictable intervals, and there wasn't yet the technology available to give them a lifespan longer than the average human's. This was suitable for fighting the barbarian human armies of the Age of Strife, but not for the long, star-spanning campaign of the Great Crusade that the Emperor set as his next goal, so as soon as he had enough territory to set up a secure laboratory and enough scientists to man it, he set to work on the next generation of warriors: the Primarchs. Some of their defects were intentional, as the harsh conditions of Terra meant that they were psychopaths that the Emperor did not want to use forever. .
Eventual Fate
The Thunder Warriors were not what He envisioned as the defenders of humanity, they were blunt tools of destruction and little else, far inferior to the bespoken Custodians that accompanied them. It is very important to note that the Astartes were already in circulation before the Thunder Warriors were "retired", the First Legion had already been established and Proto-Legions all the way up to the XVIIIth were seeing active combat. The Emperor saw no great loss in removing them for good now that they were obsolete. The Imperial records indicate that the last of the Thunder Warriors all perished at the Battle of Mount Ararat, which is strongly believed to be a cover story. Though the details are scarce, the battle did actually happen and there were survivors, hinting at a possible betrayal and a cull instigated by the Custodians.
However, contrary to this popular viewpoint, the retirement of the Thunder Warriors may not have been a betrayal and perhaps something less dickish. The Thunder Warriors leader: Arik Taranis is vague on the details and admits he holds no ill will towards the Emperor for what he did. Another survivor: Dahren Heruk still considers himself loyal to the Emperor and holds to his pre-unity oaths. More interestingly, when Heruk rescues a Custodian from a group of Alpha Legion hiding on Terra, the surprised Custodian thanks him and seems prepared to leave him where he is, at least until Heruk halts him to request and be granted an "honoured death" (basically a mercy kill with a blade through the heart). Like Arik and Ghota, Heruk's group of Thunder Warriors are all failing physically, filled with cancer, and required to undergo regular organ transplantation just to stay alive; and are failing mentally, wracked with hallucinations of the Unity Wars like soldiers with PTSD. It may be that the last of the Thunder Warriors went willingly to their ends at Mount Ararat at the very height of their eminence, knowing that they could go no further than that. The alternative being eventual madness and inevitable biological collapse, coupled with being rendered obsolete in the shadow of the newly rising Astartes; Heruk remembers the battle of Mount Ararat to have been glorious, and he and his companions all believe that they have simply lived too long.
The so-called "cull" wasn't the end of the Thunder Warriors, enough of them managed to escape and spread throughout the Imperium. During the Great Crusade, there was an insurrection on the asteroid prison colony of Cerberus. Though the Thunder Warriors were not the only members of the rebellion, there were enough of them to form a group calling themselves the Dait'Tar. The Emperor did not like that and sent an army of War Hounds to crush the prison riot with impunity. The Astartes having gotten bored with maiming and killing prisoners not worthy of their challenge, found themselves some Thunder Warriors grouping up in a defensive position. Instead of just showering bullets on the surviving Warriors like a more sensible less manly Legion might have, the War Hounds rushed in and engaged them in close combat. But Thunder Warriors are basically mini-Primarchs without the immortality, and were able to claim three to four Marine kills in melee for each Warrior that went down. After five hours of carnage and RAEG of old vs. new, the Imperial Army forces waiting in orbit got bored and decided to join the party, but the party was already done and left nothing but Thunder Warrior corpses, lots of War Hound corpses, and lots and lots of regular prisoner corpses cut down apparently trying to escape the melee.
Others descended into the Underworld of Terra, taking roles amongst the criminal element or eking out a life as Gladiators. One group managed to steal an Astartes progenoid gland to extend their lifespans, and went into hiding, so far not to been seen again.
Capabilities
It is generally accepted that Thunder Warriors fall somewhere between Astartes and Custodes on the scale of who-beats-who. Though their exact capabilities are not covered in any great detail.
During the Unification Wars, every Warlord and his dog had access to some form of advanced technology, whether through cyber-augmentation, bio-alchemical processes or genetic tampering, the Thunder Warrior was supposed to superior to all of them. Claiming "unparalleled superhuman physical power, gene-programmed resistance to environment" and most notably: resistance to "psychic attack".
This last one is particularly notable, since while his Custodians and Astartes are possessed of some impressive willpower and determination, as well as a lot of hypno-indoctrinated mental safeguards, they seem to be no more or less able to resist psykers than any other human. What's more is that we can possibly narrow this down even further and credit it to the Emperor directly, rather than any form of genetic/biological upgrade. Merir Astelan and the first 5000 Space Marines all had similar psychic wards build into their minds granted from the Emperor himself, manifesting as a golden aura that seems to make psychic compulsions have no affect on them, and giving telepaths headaches for trying to read them. This is probably why the later Astartes don't share this capability, as the Emperor probably didn't have the time to mind-lock every soldier under his command.
In any case, a few things are clear: while they do have additional organs (seeing as they had to harvest organs from their own dead rather than from normal humans to prolong their lives), they don't have a Black Carapace; the original Mk. 1 Thunder Armour was not designed to function with one, so Thunder Warriors wore their heavy armour like any mortal would, rather than interfacing with it like a second skin. They also don't have a Betcher's Gland; one of Dahren Heruk's companions, Vezula Vult, forgot about this when he tangled with a group of Alpha Legionnaires in the slums of Terra, getting sizzled in the face by acid because he wasn't paying attention. Thirdly, they don't have a secondary heart as seen with their tradition of "Honoured Deaths" with fellow warriors mercy killing mortally wounded comrades with a blade through the heart. Fourthly, and probably most importantly, they don't have a Progenoid Gland, which for Space Marine is the eau de Astartes, making them everything that they are, managing their physiologies from day to day as well as providing the means to make more of them.
However, that the Thunder Warriors are physically superior to Astartes is not really in doubt, with each conflict in the fluff between them generally resulting in it taking several Astartes just to bring down a single Thunder Warrior, and even in these reported situations, the Thunder Warriors in question are long past their physical prime and are undoubtedly in the throes of genetic degredation, so who knows that they might have been capable of at the peak of their power. They might have been directly comparable to Custodians, who knows?
Ironically, if He had made a few more regiments, or at least kept the survivors, He may well have defeated the Machine Cult on Mars (who are behind everything and needed to die) and all the grimdark the Imperium now deals with would be at a minimum instead of up to eleven. Hindsight is twenty-twenty.
Arik Taranis
The first badass granddaddy of all Space Marines and Thunder Warriors who would make Girlyman and Dante envious for his existence. He was known to win lots and lots of battles for the Emperor, survive dozens of suicidal fights in which countless Thunder Warriors (who are stronger than space marines) would die, especially the last battle in which he barely made it to victory with his comrade. The Emperor, knowing the Thunder Warrior's limited lifespan would not serve any good for his future conquest, decided to abandon the Thunder Warriors by arranging an utter betrayal with Arik that would make Horus look like a saint. As a result, Arik was forced witness the killing pretty much all his surviving comrades while according to the record, it would be reported that Arik and the Thunder Warriors had been honorably slain during the last battle. Despite the betrayal, Arik had no sense of hatred towards the Emperor, for he knew that Thunder Warriors would only serve as an impediment to their future cousins.
Anyway, Arik would later hide among Terra's population with his large, obvious body and become the Kingpin of Terra. He started out in the Petitioner's City of the Imperial Palace, gang raping his way up the food chain with his only remaining Thunder Warrior buddy, Ghota (who, as described in The Outcast Dead, is an overgrown, cantankerous motherfucker with eyes like the rage zombies from 28 Days Later, apparently a side effect of the Thunder Warriors' painful physical degradation) and at least thirty other survivors, until he was effectively the big cheese of gangsters. Calling himself Babu Dhakal (or at least everyone else called him that... the man was crazy, but he wasn't crazy enough to address himself in the third person), Arik expanded his control until it encompassed Drugs, bitches, gambling, weapons, and even toilets--the absolute madman! He would go on to control criminals and heretics alike, causing the Adeptus Arbites to rage quit because they couldn't handle Arik's near swaglord level reputation and rampant (to the point of being fucking psycho) badassery.
Fortunes came to him when he found a secret lab buried within Terra and finding the lab is about as common as finding a buried starship on post apocalyptic era on Terra however. This, coupled with the scientific knowledge he somehow got from the Emperor, would help him to genetically modify and increase his life span (but not really enough to keep him alive -- until the Horus Heresy came around).
The Outcast Dead incident
When heresy first became a thing that happened in the Imperium, Arik managed to get a functioning Progenoid Gland from one of the outcast dead space marines (it's unclear which one, but they probably didn't mind too much--being outcast and dead and all). He was able to extract at least some of the information he needed to replicate the gland, and was then able to plant a functioning copy he had created within his own body, as well as Ghota's. Since GW never expanded their storyline, we can assume that either they have successfully outlived the Emperor and become more rapey, pillagey, and space piratey than the Dark Eldar, or that the result of all their careful work was that they up and turned into uber powerful heroes of the Imperium... we have no idea. They may have just settled down on Terra.
Considering that Arik had no problem with the Emperor disposing of the Thunder Warriors (likely viewing himself as a willing tool for the Emperor anyway), that he now has a progenoid implanted, and the physical capabilities of Thunder Warriors, it might be possible that Arik founded the Sons of Antaeus and other chapters during the cursed 21 founding. Especially if the progenoid's phsyical enhancement abilities repaired his body and if he acquired Space Marine psyho-indoctrination technology. This would readily explain everything about the Sons of Antaeus from their size, strength, and durability to their distant attitude and reluctance to communicate with other Imperial forces. Getting ships and wargear would have been easy since the High Lords likely revere the Thunder Warriors like everyone else.
Arik Taranis Facts
Despite being outdated and retired, Arik has demonstrated some epic Primarch-level shits:
- He knocked down the "Azurite Tower" during the Unification Wars, as well as earning tons of fucking countless titles.
- He survived the deadly last battle that nearly wiped out his army, meaning he gets shit done even if it almost killed him.
- His Thunder Warrior subordinate Ghota can fight 5 elite Astartes to a stalemate even killing one of them, yet Ghota bows to Arik. Let guesses hop at how OP Arik is.
- Arik decoded the gene-seed within some dirty drug lab, which something only the Emperor is capable of out of every living thing in the galaxy. He could probably be the greatest Apothecary in the Imperium, next to The Emperor and Eldar goddess Isha (and Apothecary Fabius if he hadn't turned bad).
- Atharva, one of the outcast dead and a powerful Thousand Sons psyker observed Arik and commented that he has an: "aura too bright to look upon. His presence had a gravity all it's own, demanding all attention and fear." and he "could barely stand to turn his psychic senses on him for fear of being overwhelmed.". Keep in mind only the Emperor and the Primarchs has that kind of aura, yet Arik, a Thunder Warrior has that kind of aura surrounding him.
- He is named after Taranis, the Celtic god of... thunder. So yeah, the guy is literally a god-tier (thunder) warrior.
- also while it is not confirmed this random anon could see him possibly being involved with the creation of the primaris marines which are basically are half way to being thunder warriors