Star Wars has always been a universe built one block at a time. George Lucas didn't imagine the worldwide phenomenon the franchise would become when he was creating the original film. As a result, some elements feel a bit out of place with others. For example, the name "Sith" has taken on a second meaning with the introduction of a species that originated the term.
There are a lot of alien species in the Star Wars universe. Most of them are background details, but a few offer substantial unappreciated contributions to the galaxy. Those contributions aren't always positive. A universe with unquestionable pure evil will have some bad apples.
Who are the Sith?
Around 100,000 years before the events of any Star Wars film, the Sith evolved on a planet called Korriban. Their skin is red, typically darkening as they reach adulthood. Their faces are predatory and intimidating. A pair of tendrils hang from their cheeks to the bottom of their jaws. Bone spurs add sharp points to multiple areas around their bodies. They have dangerous claws on their hands and feet, as well as pointed teeth and small horns. Their unique physiology grants them inherent gifts. All Sith have vastly improved senses when compared to humans, with some being able to hear many times better than other species. The Sith's greatest asset is their innate connection to the Force. The Sith have the highest rate of Force-sensitivity of almost any species. They have a symbiotic relationship with the Dark Side of the Force that allows them to do things no other race could.
What was Sith Culture Like?
The Sith were always at war with each other. Their behavior incorporated primitive rituals such as the sacrifice of sentient beings and overwhelming xenophobia. The hierarchy of Sith society is set in stone. Their roles are so heavily stratified that they tend to become subspecies. The lowest caste were slaves, who were frequently used as sacrifices. When their masters died, they'd be buried alive in their tombs. Above the slaves were warriors, engineers, and priests. The physical traits of each class provide them with benefits toward their primary purpose. There was no mobility between castes. Sith would almost always die as they were born. The Sith created unique weapons and artifacts with their mastery of the Dark Side. Much of their history was recorded, but its alien perspective made it incomprehensible for most readers. Violence, competition, and strife were the basic facts of life on Korriban. Only one ruler was able to unite the warring Sith, but his reign completely upended large aspects of Sith society.
Around 28,000 BBY, a Sith named Adas came to power. His charcoal-black skin marked him as unique from the moment he was born. Adas was groomed to be the chosen one of the species. As he aged, he demonstrated an unprecedented talent for combat, engineering, and magic. Adas became the Sith'ari, the perfect being and undisputed king of his people. His ascension required bloodshed, but his rule lasted more than 300 years. The Sith began to believe that Adas was immortal and that his empire would last forever. Korriban was invaded by the Rakatan Empire, who possessed advanced technology far beyond that of the Sith. Luckily, the Sith had Adas, who led the battle with his massive black battle ax and drove off the invaders. Adas seized the Rakatan's advanced technology, finally granting the Sith access to hyperspace travel. The fight cost Adas his life, but it allowed the Sith to spread their empire across the galaxy.
How did the Sith Get Their Name?
Adas' death was catastrophic for the Sith. They returned to their lives of constant war, now escalated by the introduction of more powerful weapons. Korriban swiftly became uninhabitable, forcing the Sith to pick new homes and treat their old planet as a mass grave. Though new overlords took control of the Sith, there was never another Adas. However, a new caste of leaders appeared from an unexpected place. Korriban became the new home for a group of Dark Jedi, ambitious users of the Force who decided to tap into the Dark Side in pursuit of power. The Dark Jedi were shocked to find an indigenous population of Force-sensitive warriors and wizards. The Sith opened the Dark Jedi's eyes to the power of the Dark Side. Despite their innate connection to the Force, the Sith were no match for the Dark Jedi. The renegade knights declared themselves the new Sith Lords of Korriban and claimed the name of the people they ruled.
The Sith race raises a lot of interesting questions about the nature of the Force. A species of beings who were inherently connected to a primordial evil would have no choice but to live hard lives of strife and violence. There is so much to explore in the Sith Empire, though most of the existing material was de-canonized. The Sith got their names from the unique species that gave them their crash course in darkness, but there's more to them than evil magic.