I just thought of a way that you could have the Sith be introduced into post-Ruusan, High Republic setting; you don’t use the Rule of Two Sith.
How many Sith Empires were there? You have the original one which fell in the Great Hyperspace War. Then Exar Kun’s Empire, Rehab’s and his followers, Vitatie’s Empire, the New Sith Empire. That’s thousands of years of dark side practitioners all across the galaxy. Uncountable tombs on lost and dead worlds, containing the teachings of the Sith.
Something that I do not believe has been emphasized is how unnatural the Sith are. They are not bound by the will of the Force. They manipulate it, abuse it, rape it. Engineering biological horrors like the Rakghoul, Terentateks, Leviathans. Building weapons like the Dark Reaper or the Sun Razor. Transforming themselves into inhuman monsters like Sion and Nihilius. You could go Lovecraftian with the Sith.
Have explorers land on an unknown planet and discover an ancient temple. Strange symbols line the walls depicting a humanoid figure bound in chains, other figures kneeling before it. Eventually the explorers find a tomb, a tablet in aruebesh begging the reader to keep it shut. A foolish explorer opens the tomb and then is shredded by flashes of red light. A figure floats out of the tomb, a wounded being encased in a burning suit of metal, wielding a lightsaber.
A Sith that fed on his own suffering to keep himself alive, only to have over a thousand years of being entombed alive drive him mad. The explorers have to find a way to seal him in the tomb or kill him. Otherwise the thing that was once a Sith Lord would escape to cause untold suffering throughout the galaxy.
