Mejjatrythe

The Mejjatrythe were a species almost solely focused upon the harvesting of other sapient races. They built massive sublight ships dedicated to reaching other inhabited systems and harvesting them, and fired gargantuan mass drivers at those they thought were out of their reach. The reason for being so relentlessly xenocidal (although this is not entirely accurate, given what they do to those they "harvest") is unknown*, but the massive societal shift that resulted in it occured during a period of time they called their "Enlightenment." By the time of their contact with Red One, they had rendered four other sapient races extinct.

Red One encountered this species around a thousand years into the Long War, and prevented the genocide of a fifth planet-bound civilisation (around the technology level as modern Earth) by one of their torchships. With the knowledge of their nature and the lack of anyone else nearby capable of doing anything about it, she made it her mission to exterminate them, before they could develop shock drive technology and become one of the galaxy's larger horrors. The remains of their cities and space stations would later be discovered by another spacefaring race, who would term the one responsible for all the destruction the Uncreator.

* although it is at least hinted to have been caused by outside influence, most likely Devoured/Devourer