More lore, because I'm bored.
Multi-User-Mainframe: Database // Tak Records // The Shroud // Symbiosis
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By far the oddest behaviour of the Shroud, but also the most beneficial to us.
Symbiosis is a process by which the Shroud implants itself into the body and soul of another being. While this does not sound like a good thing, it is surprisingly benevolent.
It begins, as things often do with the Shroud, with an energy-embryo. I'm not yet certain where it comes from, but I can say that prolonged physical contact with any Shroud lifeform allows it to transfer the embryo into another host.
This host is inevitably a living creature that possesses a soul with a connection to the Warp above a certain strength. Measuring this is difficult for us, requiring extensive examination with specialist tools, but the Shroud is capable of doing so easily through an unknown method. Most Humans measure above the threshold.
Once implanted into the host, the embryo grows within the soul and the body of them, drawing energy from the Warp through the soul's own connection. The first stage of Symbiosis is marked with the host's decreasing need for food, drink, and sleep, as well as a quickening of healing rates and a demonstrated protectiveness from Shroud lifeforms.
As the embryo grows, it is capable of drawing more and more Warp energy through the soul, consuming it to fill its own growth. A small portion of the energy it gathers is transferred into the host's body, which is responsible for the host's decreased needs. At this stage, the embryo is an immaterial entity, existing mostly as an energy matrix within the host's soul and body, possessing little to no capability on its own.
Stage two begins once the embryo is sufficiently grown that it can indefinitely draw in more energy than the host needs to stay active. At this point, the host needs no food or drink, and sleeps only to maintain their mental health. It's at this stage where it is possible to see physical signs of the symbiosis, namely a soft blue light radiating from the chest of the individual in question. This light is not particularly strong, and emits from a small portion of the skin at the approximate center of the chest. It can be covered up by clothing, and in the early period of stage two, even by sunlight.
At stage two, the Shroud is no longer as protective of the host. This is because the Shroud Symbiont is itself capable of protecting the host should danger appear; mostly by physically manifesting energy around the host, capable of forming either armour or weapons, as well as enhancing strength, agility, and reaction speed. Later on, it may also gain the ability to attack at range, flinging bolts of energy at any aggressor.
Whatever excess energy is not used to protect the host and grow the Symbiont is transferred to the nearest Entity. As the Symbiont can draw from the Warp, this effectively turns the host and Symbiont into a slow but endless font of energy for the Shroud.
Stage three is the final stage of Symbiosis, occuring once the Symbiont reaches the maximum amount of energy it can draw from the Warp through the soul of the host at any given point in time. A person's soul is only so capable of drawing from the Warp, and while the Symbiont is capable of artificially increasing this amount, even the Symbiont can only push it so far. The Symbiont, however, is fully grown, developed to utilize the totality of energy it can draw from the host.
For most normal Humans, there is not a particularly large difference between stage two and stage three. The only visible part of it is the strength of the light they emit, which typically does not grow particularly strong. When the Symbiont manifests, it will only be able to create a body of slightly larger than Human size, though it will possess strength and agility far surpassing that of the host. Most Symbionts will likely still appear as armour and weapons, rather than a fully realized creature in its own right, providing protection and enhancement for the host. Even ones that do appear as full creatures will not be able to leave the immediate vicinity of the host.
For certain beings, particularly Psykers, with an innate and stronger connection to the Warp, stage three can be a dramatic difference. The Symbiont will keep growing based on the amount of energy it can draw, and the Symbiont of a stage three Psyker is typically extremely powerful. For approximately the same reason, this makes Psykers the favored host of the Shroud, as their strong and particularly connected souls enables the Shroud to draw in vastly more energy from them.
Symbionts of a stage three Psyker can materialize whole bodies with ease, at sizes significantly larger than the host. It might also be able to manifest multiple bodies, as well as weapons and armour for the host. A stage three Psyker Symbiont is a force to be reckoned with, even at quite a distance, not only from the physical form of the Symbiont, but also the host's increased reaction speed, strength, and agility.
Aside from the physical benefits of Symbiosis, the host also enjoys a number of other effects. Most notably, the host becomes effectively immune to Warp corruption. The Symbiont either absorbs or repels Warp energies, and while this could be theoretically overwhelmed with enough power, the amount required to overwhelm the Symbiont would immediately kill the host and destroy the soul. Hosts are, further, also resistant to most forms of direct psychic assault, as the Symbiont will either absorb or repel the energy making up the attack. Hosts also effectively jam most forms of psychic perception, precognition included, on account of the constant Warp-disruption the Symbiont causes.
There is also, of course, the benefit provided by the Symbiont. At stage three, Symbionts have bonded tightly to the host, and understands the host's thoughts, emotion, and will. This allows the host to direct the Symbiont as they wish, giving them access to the breadth of abilities available to the Symbiont. As each Symbiont is unique and evolves in response to the host, so too does the abilities available to any given host vary. Symbionts of Psykers have a tendency to display particularly powerful abilities, across a wide range of subjects.
One note of Psyker Symbionts, however, is that a Psyker, because their Symbiont is monopolizing all Warp Energy that can be gained through the Soul, effectively loses their Warp-based abilities. For latents who have not yet figured out how to use their abilities, this makes no real difference. For realized Psykers, this loss instead trades personal Warp-based abilities for general protection from the predations of the Warp, and, in time, learning how to use Shroud Energy to replicate the effects of their original powers. Most consider this tradeoff worth it, especially since Symbionts can continuously draw more energy than a Psyker alone could, typically making them both more powerful and more focused.
The time it takes to progress through the stages of Symbiosis is dependent on the strength of the Warp connection the host possesses. At lower strengths, it may take years to progress to stage two, but only a month or two afterwards to progress to stage three. At higher strengths, especially for Psykers, it can take only weeks to reach stage two, but decades to reach stage three.
The stronger the connection, the stronger the Symbiont and host, and the longer it will take to reach the height of their possible power.
The Shroud does not attempt Symbiosis on anything with a Warp connection below a certain level, presumably because the amount that could be drawn from Symbiosis would be lower than what it would require, thus presenting an energy-drain, to no benefit.
I measure the strength of the Warp connection in grades. Grade zero, and below, the Shroud will not join with. The average Human falls within the range of Grade 1 through to Grade 3. The average Psyker falls into the range of Grade 6 through Grade 12. Particularly exceptional, or particularly poor, individuals may fall into a greater or lesser ranges.
Of course, that is only the averages for our world. Others might be different, higher or lower, and both cases certainly only apply to Humans and Human Psykers.
All in all, interesting and intriguing. And, about the only thing that saved our lives, on this world.
An example of the energy armour formed by the Shroud:
This armour still resembles parts of the current typical Shroud lifeforms, if only in the glowing blue lines and semi-organic dark greys.
Another example, this time a pair of fully-manifested Symbionts from two exceptional stage three twins, at Grade 5, though not actually psykers:
Note how Human these Symbionts look. This particular pair have had their Symbionts since early childhood, and the Symbionts have adapted completely to them, to the point of even having adopted apparent sexual dimorphism, based on the pair. Again, aside from the glowing blue light, the Symbionts no longer resemble typical Shroud lifeforms at all, having even adopted a seemingly partially mechanical nature, which I believe stems from the fact that the one who raised them was a Man of Stone.
It is fascinating, how completely the Shroud can change itself. Humans are the only beings I have seen the Shroud engage in Symbiosis with, and yet, it has resulted in lifeforms of many different shapes and sizes. What more can the future bring?