Origins

Sarnak the Immortal was an immensely powerful star-god, feared throughout the outer rim.  Sarnak started as an experiment of Cosmid, inc. on the planet Ovid in the Tartarus sector.  Sarnak escaped containment in GSC 517.

Originally intended as a living source of transplant organs for the residents of the experimental garden-world Ovid, the nascent star-god rapidly developed an insatiable appetite for the internal organs of a number of species, and managed to absorb several scientists and lab technicians into its own biomass before planetary security forces were sent to destroy it.  By the time  they arrived, Sarnak had grown to proportions that prohibited its destruction without irreparably damaging the engineered biosphere of Ovid.  Cosmid decided at this point to attempt to kill Sarnak by lifting it into space with armored cargo-barges and tractor-beams.  Sarnak survived, and even thrived away from Ovid, which it had imprinted as its spawning ground.  Sarnak then left in search of sustenance, destroying those who would hunt it with an impressive array of biological weapons.

Composition

While Sarnak was composed primarily of engineered biological tissues, it also contained a substantial cybernetic neural net, and a number of interior chambers for its favored devotees, constructed from the remains of the laboratory where it was created as well as several hapless ships that crossed its path.

Sarnak's worshipers regularly delivered living sacrifices to their god in order to grow its immense body.  Some of the more technically savvy among them also worked on maintaining and expanding Sarnak's cybernetic components as well.

Cultists of the Immortal Father of Organs regularly dedicated one or more of their organs to the god as part of their initiation ceremonies.  These were collected by acolytes and absorbed into Sarnak's mass.  Followers who dedicated organs got semi-cybernetic, semi-biological organs cloned from the body of Sarnak that served the purposes both of the dedicated organ, but also those of the cult leaders.  

Some of Sarnak's most zealous followers volunteered to be bodily absorbed by the star-god. 

Cult

The cult of Sarnak the Immortal spread along the galaxy's outer rim, far away from the core-worlds, where Sarnak could avoid the might of the larger militaries in the galaxy, while preying on the settlers of relatively undefended frontier worlds. 

Cult leaders skilled in the dark arts of both biological engineering and rabble-rousing would scout out a prospective inhabited world, and infiltrate it gradually.  Often they would introduce a pathogenic agent (engineered viruses or nanobots were popular options) into the planet's environment before offering to "help" the population by providing medical care in exchange for their goodwill.  The clinics they would set up would offer not just medical aid, but indoctrination as well. 

Often the leaders of the new cult would conspire to assassinate or disgrace any other doctors or biologists who may have been present in the community.  Those in need of new organs would be required to join the cult of Sarnak before surgery would be performed.  At some point the leaders would introduce an "Organ Plague" to the planet, which would temporarily suppress the function of one or more of the victim's organs, in order to win converts. 

Sometimes, the vanguard of the cult would arrive in a place where Sarnak could not follow soon enough to receive the dedicated organs, in which case they would be either implanted into a cult champion (a high-ranking leader), or placed into a special bioreactor with various cybernetic components to create an Avatar of Sarnak, a powerful, engineered entity that would serve as an intermediary between the local cult and Sarnak.

Death of the Immortal

Sarnak was finally destroyed by the Galactic Republic in GSC 26352 / YE 25437 when it returned to Ovid to spawn, directed by a deep-seated instinct.