The T'zca (Nynosuke) (Patreon)

The Nauzin were barely a spacefaring species when they were forced to escape their solar system, their enemies had made a dyson sphere around their sun, rendering their world uninhabitable. 

Their fleet traveled the dark for decades in search of a new home, and as nutrition dwindled many ships turned to more desperate measures: the process of decarbonization. A vast biological and physiological conversion that removes the necessity of organic sustenance and enhances their blood's ability to sustain them. 

While effective in the short term, this process was... flawed. Though the viability of the decarbonized, or T'zca as they were called, blood's nutrition now lasted weeks, it merely prolonged their resource strain, and created an entirely new demand:

More blood.

Gradually the T'zca process became mandatory across entire ships as many leaders took this bold step to conserve their resources and encourage their people to do the same. And when the blood banks drained, they took infusions from prisoners. When the prisoners became fewer, harsher laws produced more, and more still when many rose to protest the coming feast.

Soon it was a choice: join the T'zca, or be harvested for blood.

Some ships rebelled, some were retaken by the unaltered Nauzin, and the civil war raged for months. Soon, though the conversion was promised to many, the T'zca saw all remaining Nauzin as a blood source.

Centuries later they're near mythical, a boogeyman to uninformed spacers. Many T'zca have broken from the fleet and pass off as Human cyborgs, Sadiriaks, or pale Narns in civilized space. But somewhere, cloaked in a nebula before the edge of dark space, lies the T'zca fleet, sending periodic raiding parties to bring back prisoners for blood and entertainment.

(Credit to WeresharkofDaytona on the server for some idea bouncing for this.)