fluff

Small in size but extremely durable due to their dense shells evolved from their natural habitat in the deep trenches of Cephalon VI the Cenoceroids are a formerly symbiotic species that survived alongside the large bottom feeders of their planet that turned more parasitic over the centuries as the Cenoceroids began controlling the larger and dumber species through psionic controls. As their powers expanded so did their ability to grow as a species and the Cenoceroids took control of the few warm parts of the planet and abandoned the rest to the wilds. Biologically the species is fairly unimpressive other than their natural telepathic abilities allowing transfer of thoughts between others of their kind (originally a warning system evolved to hide from the predators that can shatter their shells) and the dominance of lesser minds through physical contact (opriginally a kind of psionic stun to catch prey that developed into motor control and minor hypnosis and while it sounds impressive anything more than a short stun isn't going to work on anything smarter than a gorilla and even a gorilla could probably be trained to repel it given time). Other than that they are a predatory species but due to their size mostly eat small prey such as fish or small crustaceans. Physically the Cenoceroids are about half a metre across at their widest
Technologically the Cenoceroids were at roughly the nuclear era of technology with focuses on improving their own biology - Nautilus subspecies are notoriously slow to evolve and on Earth are still using simple eye structures so the Cenoceroids like to help nature along a bit and until recently used elective surgeries to alter their capabilties but over the last century have begun delving into the realms of altering themselves at a genetic level and so far have hit the point where they can survive outside in multiple atmospheres - until the tragic loss of a research expedition of an M Class race that became the guests of honour in one of the metropolises of the Cenoceroids and were interrogated for months on the functionality of their technology while their ships were dissected and converted to work with the simpler technology of the Cenoceroids as well as allow the internals of the ship be completely submerged.
Race’s motto: Control to Survive
Home Planet: Cephalon VI
Home planet’s fluff: A hostile surface world of thin atmosphere, chilling winds and radiation, and a thick layer of Ice. The only Life you'd see are the slow crab-like scavengers moving close to the ground and the lichens growing around the volcanic vents that break through the ice and allow access to the Oceans below the surface. The Oceans themselves were once a wild ballet of dominance as various monolithic predators devoured eachother and the smaller species until many millenia ago the Cenoceroids (an at the time intelligent but weak prey species) developed a symbiotic relationship of control with one of the larger prey species and figured out how to dominate the minds of at first the simple migratory species and eventually the powerful predatory ones. Now most of the oceans are left to the wilderness but the biologically valuable locations around the aforementioned volcanic vents are controlled by the Cenoceroids and until geologically recently it was a very simple area but with the safety of dominance and the plenty of food the Cenoceroids were able to grow in technology and developed a society up from the stone age all the way to the nuclear age and built great metropolises around the various concentrations of volcanic vents around the planet.

Tech is based around biology of mindcontrolled slave species