@Durin
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Storm Zealots
Origin
Throughout the history of Bel-Caerel, the Kingdom has fought Chaos Space Marines on numerous occasions. When Ilfeliare came to power, she put in a policy where the captured gene seed of Space Marines would be locked in stasis vaults rather than destroyed. She hoped to purify the gene seed of taint and then use it to make her own Space Marines, but all attempts to do so ended in failure. Despite the failures, she kept stockpiling gene seed and locking it away, hopeful that at some point in the future, she could acquire divine lore that would let her purify it.
This policy would end following the conquest of the Kaerae Worlds and the alliance with the Arctodons chapter. To begin with, all the gene seed of Loyalist chapters - stolen by Chaos Space Marines as is there wont - was relinquished to the Arctodons. This bought Bel-Caerel additional goodwill with their ally while also strengthening that ally. But that wasn't all.
Before, the creation of Space Marines was limited not only by the impurity of the gene seed, but by the lack of instruction of how to use it. Now that Bel-Caerel had Space Marine allies, the Kingdom could theoretically learn how to make Space Marines of their own. With the galaxy in the state that it was, it was imperative to use the massive stock of gene seed accumulated over thousands of years to make more Space Marines, traitor lineage or no. However, since Ilfeliare lacked the ability to ensure the level of loyalty that was required from Space Marines, she gave the gene seed to Karzarot, who could.
Getting the Arctodons to help was a tricky thing. They were being asked to travel into a warp storm and make Astartes out of traitor gene seed for a soul-eating warp entity. It was a difficult ask. Ilfeliare vouched for him, which helped, but also cast suspicion on Ilfeliare herself. A few Arctodons ventured into Karzarot's home to gain more information on the god, and it did corroborate the storm and sun gods' words, but Chaos or not, he still ate human souls.
At first they refused. They had been given the tomb of a Living Saint to protect, but that tomb was inside a warp storm where Karzarot ruled supreme. The god claimed he'd allow the Space Marines to remain and pass through the Drirh Cyclone if they refused, but the Arctodons had no concrete reason to believe his words. They refused to make him Space Marines; better a destroyed tomb now than a destroyed tomb later with enemy Space Marines in addition. Upon receiving the news, Karzarot grumbled, returned the gene seed to Ilfeliare, and turned his focus to other matters, but kept his word.
A century had gone by and the status quo remained unchanged. The gods of the Firmament pursued their respective interests, the traitor gene seed remained locked away, and the Arctodons continued to guard the Living Saint's tomb without issue. The status quo of the galaxy remained unchanged as well, but as the status quo was "everything is bad and getting worse, at an occasionally accelerating pace", that was not a good thing. Ultimately, desperation drove the Arctodons to contact Karzarot and accept his original offer. Humanity needed more Space Marines to protect it.
Thus began the history of the Storm Zealots, the traitor-descendent Space Marine chapter sworn to the storm god Karzarot.
***
Training
The training of the Storm Zealots is gruelling, even by Space Marine standards. The training process took most of a century of evolution and fine tuning before it became non-lethal enough for the chapter to begin producing a notable surplus of gene seed.
The bulk of the training isn't entirely unorthodox for loyalist Space Marines. They were trained by orthodox Ultramarine successors and are raised largely in the manner dictated by the Codex Astartes. Their training does diverge in a few significant areas though.
When travelling through the Wyrmhome, Karzarot's warp storm, their minds do not get the full protection against its horrors that the rest of his followers receive. With their minds so close to the screaming edge of the warp, they receive visions and compulsions pushing them to lax their minds and submit to Chaos as their primogenitors had. Every time they must resist that call with faith and will, and if they surrender even slightly they will be immediately terminated. The protection they receive recedes as their training progresses, putting greater strain as time goes on and forcing their minds to reach ever greater levels of spiritual fortitude.
Once the neophytes finish their training, they regain the full measure of Karzarot's protection and receive their power armour, but are not yet full Battle-Brothers. They instead receive a rank called Initiate, and will retain that rank for 10 years. Unlike other chapters, the Storm Zealots remove both progenoid glands as soon as they have matured. Once an Initiate's second progenoid gland has been removed, they will undertake the Rite of Becoming.
The Rite of Becoming begins with the Initiates going on a raid to capture orks or heretics and take them to the chapter's fortress-monastery within the Wyrmhome. There, the captives will be sacrificed in a ritual to summon a number of Storm Dragons, Karzarot's divine servitors, who will then possess the Initiates before promptly leaving and returning to the Warp.
The purpose of the exercise is to provide a final layer of protection to the Initiate's soul by inoculating them against exposure to the Warp. From this point on, he can be trusted to remain loyal and pure even under the most arduous of circumstances, despite their dark lineage. As with ordinary daemonic possession, this form of inoculation also means the formerly possessed can no longer develop mutations through exposure to the Warp. The purpose of waiting to do the Rite of Becoming until after the progenoids are removed is to avoid exposing the gene seed to warp energy. This is something the Storm Zealots have to be extremely careful with given the malefic influence locked within the organs, and also to prevent them from being damaged.
Once an Initiate completes the Rite of Becoming, they rise to the rank of full Battle-Brother.
Initiates are sometimes jokingly referred to as Little Brothers by their older Battle-Brother siblings.
***
Organisation
During the Storm Zealots' founding, the Arctodons decided to institute the balanced, arch-orthodox organisation of the Ultramarines. It would inevitably waste the individual talents of the marines to an extent, but it would ensure that the chapter as a whole would work well together, which was considered a higher priority. It would also serve as a good base for the chapter to evolve from, to mould itself into a form that could more optimally utilise its own strengths.
In point of fact, the chapter's organisation didn't end up changing much at all. Marines were assigned to do whatever they were individually best at, so specialists still operated at a high level of effectiveness, and the standard training and organisation succeeded in making sure everyone could work well together despite their differences. (Though in reality, said differences turned out to be fairly minor; training and culture were bigger influences than blood by far.)
The only significant change to the chapter's organisation since the Storm Zealots' founding was the merging of the Chaplaincy with the Librarius.
***
Practices
Rather than Chaplains or Librarians, the Storm Zealots have Storm Priests, who combine the offices of both. Storm Priests who aren't psykers will take up sanctic sorcery, or else bargain for the gift of psychic ability from a storm dragon. Afterwards they will bind their souls to Karzarot, similar to how a sorcerer of the Heretic Astartes might bind his soul to one of the Ruinous Powers for greater control over his magic.
Storm Priests may summon storm dragons into battle.
Storm Zealots often form Pacts with Karzarot's sapient divine servitors, offering some form of payment in exchange for a gift or service. Usually they demand the living sacrifice of an ork or Chaos heretic, but it's not uncommon for a storm dragon to instead charge regular observance to themselves instead, or some other service. The most common Pacts are Pacts of Vengeance, Might, or Power, but the formation of other Pacts are not unheard of. The Pact of Peace is the most common of these other pacts, whereby a storm dragon taps into the calm centre of its being to repair a Space Marine's mind if it gets damaged. Any Pact formed between a Storm Zealot and a storm dragon must be presided over by a Storm Priest, who will forbid the formation of any Pact they see as foolish or detrimental.
Upon ascension to captaincy, a Storm Zealot Captain will be provided with a Rune Weapon. Rune Weapons are master crafted weapons covered in runes and glyphs sacred to the Storm Wyrm. Rune Weapons are special because each one is built to summon and house a mighty storm dragon in its frame, effectively making them Divine Weapons. To infuse his weapon with the might of one of his god's divine servitors, the Captain has to recite a special litany in Karzarot's divine tongue. (Typically before the start of a battle, but it can be done during it.) Rune Weapons are not meant to imprison storm dragons, and they will depart once the battle is done.
The Chapter Master of the Storm Zealots undergoes a very special ritual upon attaining the rank. His body is prepared to become a spirithost, who will house one of Karzarot's strongest immortal lieutenants, the storm dragon Fruvodim. However, the host retains control of his body. On the battlefield, the Chapter Master and Fruvodim fight as one, but outside of it, the Chapter Master is autonomous. The Chapter Master may freely communicate with Fruvodim, who has gained much experience over the course of his existence.
***
Armoury
Negavolt Weaponry
Negavolt weapons are arc weapons constructed with empathic resonance coils that charge the weapon's energy capacitors with the power of their user's hate. The warp-infused lightning of negavolt weaponry has a tendency to almost chase their targets, and is the bane of daemon engines everywhere.
Nebula Munitions
Nebula munitions are various forms of munitions made to be used by long ranged ordnance delivery systems, such as missiles, mortars, and cannons. The munitions are loaded up with extreme concentrations of exotic gasses found within the material side of the Wyrmhome. Upon detonation, nebula munitions explode in a massive radius, scattering enemy units in the primeval winds and pulping particularly close troops with pneumatic force. The gasses are highly flammable, which if lit explode in a fireball that sucks in all the air around it to feed itself in a devastating thermobaric reaction.
Thunder Drake
The existence of Heldrakes offends Karzarot to his core. In his opinion, they tarnish the good name of soul-eating warpborn dragons everywhere with their Chaosness, and he could not abide that. To persecute his vendetta, Karzarot ordered the Zealots' Techmarines to design a mechanical dragon superior to the Heldrake in every way.
To begin with, they looked at the Heldrake and noted the Daemon Engine's single greatest flaw: the neglect of its pilot. Centuries-old demigods of war lay at the hearts of the Heldrakes, but did nothing but make their roars somewhat more unsettling. It was pathetic.
The Techmarines' goal, then, was to create a Thunder Drake: a combination of Divine Engine and Dreadnought that would see the two beings at their heart, the mortal and the immortal, work together in unison. To kill Chaos. And orks. (But preferably Chaos.)
To begin with, they designed the Shell of the Divine Engine. It would house a Thunder Dragon, a creature of significant cosmic power but of merely bestial intelligence. The Shell's core would be a Dreadnought Sarcophagus, warded to avoid subsumation - accidental or otherwise - by the warp entity it will share a body with. The Shell would be designed to resist having its weapons subsumed such as what happens to the Hell Talons'. It would sport a Negavolt Blaster, an Infernum Halo Launcher, and two Stormstrike Missile Launchers. Two piston-driven claws would improve its melee ability, and the Shell would further be outfitted with Warp Rift technology so the Divine Engines create storms in their wake.
Once a Space Marine is interred within the Thunder Drake Shell, the chapter will capture a number of orks and place them in an artificial battleground. The Storm Priests will then summon and bind a thunder dragon into the Shell, activating the machine. Thunder dragons are unintelligent creatures, and so cannot be convinced by rational means to accept its new existence. It must be convinced by more primitive means, and so is provided a battlefield of enemies to destroy to assure it that its new life as a Thunder Drake will be a fruitful one.
In combat, the Thunder Drake combines the supernatural ferocity of a warp entity with the calculating focus and hyperintelligence of a Space Marine. The storm and its eye. At first the pair will struggle to work well with each other, but over time will learn to synchronise and fully bring forth both of their strengths. Few Daemon Engines are as tactically sensible or skilled with ranged weaponry as a Thunder Drake. In time, the partnership between man and warp entity becomes so close that upon the death of the Thunder Drake, the aircraft completely disintegrates, and the two entities within will fuse together into a singular Storm Dragon.
Storm Drake
After the Firmament joined the Aetheric Concordant and began trading technology, Sophont learned of the Thunder Drake. Within months it began design work for a fully mundane version, with a stated design goal of surpassing the original. While Sophont claims it is simply creating a more reliable and mass producible version of an effective unit, its extreme distaste for designs that rely overmuch on the warp is well known, and the new design is suspiciously similar in appearance and role to the original unit. This in turn prompted Karzarot to prompt the Storm Zealots to make a yet better Drake; he wasn't going to let some non-dragon god do dragons better than him. The result was the Storm Drake.
In appearance, the Storm Drake's Shell is much the same as the Thunder Drake's, but differs in a few significant areas. Its metals are created by the psyker-metallurgists of Zuntîram's World, the materials' resilience and performance above what could be achieved with technology alone. Instead of having a Sarcophagus, it has a Soul Coffin, a device originally designed to imprison daemons, but repurposed as a stomach and fuel tank. The Divine Engine's enemies go in, increased speed and firepower go out. Finally, purity seals of holy sun parchment adorn its hull, which will attract more voltaic ghosts to the Engine once it's active, providing it with a stronger voltagheist field.
Unlike the Thunder Drake, the Storm Drake is pure Divine Engine. Rather than combining a Space Marine with a warp entity, it opts to hold only a warp entity within itself. However, that warp entity is a Storm Dragon, specifically one that used to be the two pilots of a Thunder Drake. They are the ultimate pilots for these vehicles, but that's not all. The Storm Dragon produced by a Thunder Drake retains all the organic benefits of the Space Marine it once was, and these benefits transfer over to the Storm Drake. This gives the Divine Engine extra benefits such as enhanced senses and even memory-eating.
Of course, the creation of a Storm Drake is ludicrously expensive, but it's very much worth it.
As of now, the nascent arms race between Karzarot and Sophont has hit a brick wall as neither has any easy way of making their drakes better, but it remains a point of contention between the two deities.