Scourge of the galaxy since long before the earliest of the modern star empires, the Ceremorphs are not a single species in the traditional sense. Scholars debate which species began their kind, but it is merely of academic interest--they are united not by a physical kinship, but by a psychic link between their minds.
Ceremorphs do not reproduce in the traditional sense. Instead, they infect intelligent minds--of seemingly any biology--with parasites which sap individuality and free will from the host, while linking it to the Ceremorph hive mind. This tends to warp the host's body into an aberration of its former self, as the body's sole purpose becomes keeping its brain alive and useful to the hive. The parasite's genetic material is highly morphic, able to rewrite the genetic code of its host to grant it limited metamorphosis to achieve this purpose.
While the hive mind remains supreme in terms of will, the individual minds lend much to the collective--specifically, their memories and abilities. Having assimilated countless intelligent species, the hive mind has achieved mastery of science beyond any one species, and thus possesses technology beyond any of the star empires. To that end, they command a starfleet like no other, and have conquered far more star systems than the largest uninfected empires.
As if that were not sinister enough--they do not merely conquer star systems. Every system they conquer is in some stage of the long process of conversion into a Dyson Swarm. The ceremorphs do not live on planets--they harvest biospheres for useful material if any is to be found, then convert all planets, moons, and asteroids into material for the Dyson Swarm, less a relative pittance to supply their fleet. It is not known to what purpose such a gargantuan amount of energy is meant to be put.
Due to this practice, it is quite clear, with the aid of a telescope (as one wouldn't want to be close enough to see with the naked eye) where the heart of the Ceremorph empire lies. There is a patch of the galaxy consisting of more than a hundred million stars, which ought to be visible to the eye, but can only be seen with infrared and radio telescopes. They have been permanently darkened by their swarms, leaving a great void, whose existence sends a clear threat to the rest of the galaxy of its ultimate fate.