As remote and secretive as the distant star system from which they emerged, the Chiss species remains a mystery to most of the galaxy, completely removed from the Republic and the Empire, these blue-skinned humanoids evolved an advanced civilization known as the Chiss ascendancy in the unknown regions of space. Despite constant political power struggles among the ruling class, the ascendancy maintained strict controls over its dominion and the civilization prospered. This is how the Chiss developed socially and technologically in isolation for thousands of years... Until they were discovered by the Sith Empire.
During the late days of the Empire's exile, the Sith came into contact with the Chiss, and what followed was unique even in the annals of Imperial history. In every previous instance where the Empire had encountered an alien civilization with enough technology and resources to defend itself, the Empire demanded surrender and capitulation to the rule of the Emperor. Each civilization refused and the Sith conquered their worlds and killed or enslaved the inhabitants. The power struggles of the ascendancy are rarely solved with force of arms, however, and the Chiss ruling class surprised the Empire by considering the offer on its merits and asking for a summit. The Empire's military leadership, confused, agreed.
Behind closed doors the best Chiss negotiators met with a who's who of important Imperial diplomats, including the very unusual presence of the commander of the secretive Imperial Guard. Nothing has ever been publicly revealed about that meeting but the outcome is known and now wondered about by diplomats and historians the galaxy over. The Chiss would become the first official allies of the Emperor. Their armies, resources and tax revenues would be freely offered to the Empire. In return the Empire would allow the ascendancy to govern and administer its own planets, and never set foot on the Chiss homeworld of Csilla without permission. Not one political representative, not one noble tourist, not one Sith.
In the years that followed, the Chiss assisted the Sith in the invasion of Republic space and earned the Empire's respect and trust as military allies. While in Sith-controlled space, the Chiss submit to Imperial authority without question and a handful of Chiss have even integrated themselves into Imperial society. Nonetheless, the Chiss ascendancy maintains its own forces, separate from the Imperial military, and most members of the species still retain their characteristic private ways, even when working closely with their heavy-handed Imperial allies. To the Republic, the Chiss continue to be a complete mystery and the species' very existence is unknown except in high level political and military circles.
Official article on swtor.com