Star Wars' Sith Lords made their first appearance in A New Hope, as Darth Vader and his iconic suit immediately set the bar high for the franchise's villains. Meanwhile, Darth Sidious followed suit in Return of the Jedi, debuting his trademark Force lightning. More Sith debuted in the prequel films, but it was other media that brought the Sith's ancient history to light. Figures like Naga Sadow, Valkorion and many others had set an evil precedent that Sith Lords followed for thousands and thousands of years.
Star Wars' very first Sith Lord was named Ajunta Pall. He was a Jedi following the First Great Schism, and during the Hundred-Year Darkness, he declared war on the Jedi Order. Though they were powerful, Pall and his followers were defeated and exiled. As they wandered uncharted space, they discovered the planet Korriban, and using their powers, they subjugated the Sith species. Thus, Pall became the first Dark Lord of the Sith -- but it's important to note that Ajunta Pall and his followers were greatly influenced by the Sith's domineering ways. In fact, they were especially enthralled by an ancient figure called King Adas.
King Adas Was Star Wars' First Sith Emperor
King Adas ruled thousands and thousands of years before Ajunta Pall and his followers arrived on Korriban (which was later known as Moraband). Like everyone on the planet, he was a Sith Pureblood, but he looked different from those around him. While most Sith had a blood-red skin color, Adas' skin was the color of charcoal, and because of that, he was raised as a chosen being. He grew into an unusually large physical specimen, had an affinity for magic and was especially adept in combat.
Eventually, Adas decided to make good on his status as a chosen being. Using his wide skill set and his alchemically-forged battle ax, Adas began to wage war against anyone who stood in his way. Thus, he united Korriban's disparate nations and established himself as a global king. Over time, his followers started to call him the Sith'ari, which meant "Lord" or "Overlord." And his kingship, which lasted three centuries, was dubbed the "Reign of the Ax."
Why King Adas Never Fought the Jedi
For most of Star Wars, the Sith were defined by their endless feud with the Jedi, but Adas never had to deal with that. In Legends continuity, the Jedi were founded around 25,000 BBY, but King Adas reigned circa 28,000. Thus, Adas ruled before the Jedi ever existed. Granted, Adas had plenty of enemies and spent most of his life subjugating the warlords of Korriban, but the Jedi were never in his purview. Adas' only extraplanetary enemies were the Rakata, which were referenced in Andor. After he had ruled for three centuries, the Rakata showed up and warred with Korriban. Using all his strength, Adas drove the invaders away, but he lost his life doing so.
King Adas' legacy defined the Sith, and it set a precedent that Ajunta Pall would follow thousands of years later. Pall and his followers had discovered the dark side of the Force before being exiled, but they weren't overly experienced in wielding it. So, when they subjugated the Sith, they interbred with them and appropriated their domineering views of the dark side. Simply put, the Sith, including Palpatine and Vader, believed in individual power and control because of the influence of King Adas.