Sons of the Four-Armed Emperor

The Sons of the Four-Armed Emperor are a corrupted space marine chapter who, though fate or a flaw in their genetics, fell to the persistence of the tyranids to acquire space marine genetics for their own use. They were infected by a splinter group of the Cult of the Four-Armed Emperor and now masquerade as a fleet-based space marine chapter. In reality they slowly spread their infection to the very worlds they are “saving”.

The Guard of Sacrifice Space Marine Chapter
The Guard of Sacrifice were a 15th founding chapter of Ultramarine stock who eventually fell to the infection of a particularly devious genestealer cult. The chapter itself is a fleet-based chapter that crusades against the enemies of the Emperor... while spreading its infection. The first several centuries of the chapter's history were full of glories, successful campaigns, dramatic and gallant sacrifices, and what you’d expect from a chapter.

Infection
The 3rd company of The Guard of Sacrifice was dispatched to a far-flung battlefield to assist other Imperial assets. Unbeknownst to them their serfs had been infiltrated by a genestealers cult and a Purestrain Genestealer smuggled onboard. This genestealer was from a splinter cult of the Cult of the Four-Armed Emperor. The infection slowly spread until it reached the serfs assisting the chapter’s apothecary.

The apothecary had fallen ill after a battle with the forces of nurgle and, on his deathbed, was infected by the cultists. His biological defenses down, he became the first brood brother of the cult. This apothecary soon used their talents to infect other marines.

The Disease Progresses
The infected marines and their brood brothers stayed hidden for years but found that they could no longer hide their true nature. The Chaplain had been growing suspicious and when the first hybrid was born to a serf the gig was up and a blood-bath began. The infected marines, along with their apothecary leader, slew the chaplain and either killed or forcibly infected all the remaining marines.

There was a problem however, the initial infected marines were quite powerful but they were sterile. Because of this the cult relied on the chapter’s serfs to spread and grow the cult. They deposited cultists on many worlds that the chapter supposedly “saved” from other threats. They’d also take in new recruits from these worlds and infect them, making their potential scouts into fearsome infected marines.

They also slowly infected other companies of their chapter until, after a century, the chapter had been almost entirely replaced by infected marines. Several quiet battles were fought but eventually none were left untouched. The chapter has gone through many brood-cycles and produced many horrific abominations.

Life Cycle of an Infected Marine
The lifecycle of an infected marine goes as such: neophyte-scout, infected marine, alphamorph, termimorph, and then they become master strain marines if they live long enough.

Recruitment & Indoctrination
A member of the Sons of the Four-Armed Emperor begins their life as either a human or a 3rd or 4th generation genesealer. They are indoctrinated into the cult after passing a trial on one of the worlds that “The Guard of Sacrifice” save or aid. Once indoctrinated they are implanted with a mixture of space marine augments and tyranid infections that produces what is commonly referred to as an “infected scout” or “hybrid scout”. The chapter has discovered  that only the following organs and augments are required to make a new marine: the Ossmodula, Larraman's Organ, Biscopea, Omophagea, Occulobe, Sus-an Membrane, Progenoids, and the Black Carapace.

If a pure human is used they are referred to as a “Sacred Son” as they appear to be nothing more than a normal space marine unless they are biologically tested. Genestealer-based marines are referred to as “Brood Brothers” (like other members of the cult).

Neophyte-Scouts
During the scout phase of the marine’s life they are infected and mutated constantly until their augments full take and they become a fully infected marine. During this time they serve in special training squads and largely stay out the the public eye as they have not yet developed the guile necessary to fool the Imperium’s finests.

Infected Marines
The infections a member of the chapter must endure mutates them over the course of their entire lifetime. They will continue to grow and mutate, maturing almost like the various generations of genestealer cults. A trained, equipped, and mutated infected marine can go toe-to-toe with any other Adeptus Astartes in the Imperium. They hide their mutations by hiding extra arms inside their power armor, keeping their helmets on at all times (cultivating a myth that a “Guard of Sacrifice never removes their helmet”), and avoiding Imperial scrutiny.

Sacred Sons show very little external mutations and are always members of the chapter’s first company. This is effectively a “show company” to throw any imperial agents off their trail. Sacred sons never mutate past this point however.

Alphamorphs
An “alphamorph” is a veteran infected marine in their prime. They serve as squad leaders or are placed in charge of one of the brood-companies. They serve as the Master Strain’s war-council and report directly to the Keeper of Lies. They are regarded as the chapter’s finest warriors and serve until their body begins the process of becoming a termimorph. All alphamorphs are psykers, though they can only use their powers to sustain the hivemind.

Termimorphs
After many years an infected marine becomes too grotesque and inhuman to pass as a normal space marine. Such marines are given the chapter’s terminator armor or are relegated to vehicles. Their minds often lose their edge and they become more feral and monstrous, less human. They are not generally deployed when the chapter thinks they can be noticed.

Master Strain
A single infected marine leads the chapter in the same way a Patriarch runs a cult. When the previous patriarch dies a termimorph automatically begins to to mutate into a “master strain” infected marine. They encase themselves in a flesh-cocoon and emerge as a devastating and terribly powerful psyker-warrior that creates the hive-mind of the chapter.

Keeper of Lies
If a psyker, generally a magus or nexos, is converted to an infected marine they can become a “Keeper of Lies”. These are the equivalent to a normal chapter’s Librarian and Chaplain as they help create and monitor the hivemind that the Master Strain Marine produces. They are as cunning as they are powerful and are always the most crafty and guilesom members of the chapter.

First Generation Marines
The earliest generation of marines, those who were originally fully human space marines of the The Guard of Sacrifice Space Marine Chapter but succumbed to the infection hold a special place of honor. They act as the chapter’s true veteran core and are quite ancient. Their ranks are replenished by marines from other chapters who are captured by the chapter and infected. This process fails 90% of the time and is only able to be performed on those who are close to death so their opportunities to replenish their veterans are rare.

Brotherhood of the Kiss
The chapter has a large number of marines who specialize in genetics and biology and take the role of the apothecaries found in most chapters. Their inner cult is referred to as the “Brotherhood of the Kiss” as they do most of the actual infestation of new hosts.