A symbiont is a human infused with artificial intelligence. Physically, this takes the form the computer-augmention of most parts of the brain, with the substantial replacement of many parts of the occipital and parietal lobes, the corpus collosum, the brainstem, and the reworking of the spinal cord with fibre optics. These modifications allow Symbionts to integrate and process data at superhuman volume and speed, as well as allowing Symbionts to "interface", or attempt to do so, with pieces of computerised technology, simply by plugging them into their own brains, or into systems that are networked with their brains.
The Symbionts could also be controlled through their implants, at least to some extent, and many of them were designed with a "Loyalty" program that in some fashion either allowed them to be compelled against their will, or else allowed their desires and intentions themselves to be determined by others. Other Symbionts whose Loyalty programming had been disabled tended to view this "Loyalty" as essentially a form of slavery, and a counter-program, the Symbiont Liberation Matrix, was later designed to disable the Loyalty programs in Symbionts in whom it had been activated.
The Symbiont Liberation Matrix could be broadcast wirelessly over a city and achieve effect, suggesting that Symbiont brains contain wireless communications systems, or else the Matrix worked like a peer-to-peer virus, liberating any Symbiont who happened to be connected to wireless technology, then spreading from Symbiont to Symbiont until the total population of Symbionts in the area had been liberated.
The first Symbiont was created by Dr. Gustaf Brackman, and with the support of their superior cognitive capacities, the Earth Empire was greatly strengthened and the pace of technological progress surged ahead. Later on, feeling that they were being exploited for their intellectual labour, the Symbionts demanded equal rights and recognition as a distinct people within the Empire.
At this time, they were enslaved by the Earth Empire by the means of the Loyalty program designed under duress by Brackman. Brackman, profoundly uncomfortable with this arrangement, fled to a distant planet with some of his closest Symbionts in whom he had secretly disabled the Loyalty safeguards, creating the Cybran Nation. When the Earth Empire fell apart and the UEF replaced it, they kept the Symbionts as their slaves.
During the Infinite War, the Cybran Nation, chronically outmanned, practiced the strategy of avoiding conflict with the other two factions, while taking every opportunity to broadcast their Symbiont Liberation Matrix to UEF-controlled population centres, secretly freeing the Symbionts therein, for later evacuation to Cybran Nodes.
In the later stages of the Infinite War, however, the Cybran Nation adopted a more militant, radical contact policy, staging daring raids against the other two factions, bringing military force to siege and undermine UEF strongholds while simultaneously broadcasting the Liberation Matrix and inciting the local Symbiont population to revolt and overthrow the UEF government, joining their comrades-in-arms mid-battle. This policy was prompted by the realisation that the Aeon Illuminate were gradually overpowering and over-running the UEF, leading to the genocide of enslaved Symbiont populations in UEF enclaves.
It was during this period of increased militancy that the Cybran Nation learned of the Black Sun, and devised their own plan to pre-emptively end the Infinite War by using the Quantum Gate Teleporters to broadcast a Quantum Virus across the inhabited galaxy - freeing all Symbionts and simultaneously cutting off travel and communication between human-inhabited worlds, thus allowing isolated Symbiont populations a chance to make a stand on more equitable terms.
The Symbionts look mostly human, however various cybernetic components are visible on their bodies, including circuitry printed across their skin and ports or plugs in their heads for interfacing with computer technology.