The Sith, also known as Red Sith or Sith Purebloods, were a species of red-skinned humanoids that originated on the world of Korriban, before adopting Ziost as their second homeworld. Distinct for their bone spurs, facial tentacles, genetic predispositions toward both left-handedness and the dark side of the Force, the Sith species created an empire first ruled by kings and later by Dark Lords. After the arrival of the Exiles from the Hundred-Year Darkness on Korriban in 5900 BRR, interbreeding and genetic manipulation altered the Sith species, and over the generations the original species became a hybrid race of Sith and human heritage.
After the Great Hyperspace War, a group of Sith attempted to escape to the world of Malachor V, only to stumble upon Earth. There, they reestablished their society, ableit in a smaller scale, and survived until modern times. Of these Sith, one of them was a direct descendant of Tulak Hord, an ancient Dark Lord of the Sith, and it would be one of his descendants that would become Darth Revan, the Sith Lord whose actions would lead to the Jedi Civil War, the First Jedi Purge, the War of the Dark Lords, and finally the Great Galactic War.
After the arrival of the Dark Jedi at Korriban, with the usage of Sith alchemy, the human exiles managed to procreate with the native Sith, therefore making sure that their lineage survived for millenia. Over the centuries, the pure Sith race that had its origins in Korriban became virtually extinct, replaced by the Sith-human hybrids, which had the exact same appearance of their Sith predecessors, except for a few minor differences.
These Sith-human hybrids are commonly refered to as Red Sith, Sith purebloods or simply Sith.
Born from the physical alterations caused by heavy Sith magic and alchemy, the Tellurian Sith sub-species appeared during the early years of the Kingdom of Egypt. They were orginally Sith-human hybrids, but their physical appearance became altered enough that they became able to pass as normal humans without any issue, although their genetic and biological structure is the same as that of the main species. The population of Earth is 24% composed of Tellurian Sith, and one of these is the very Emperor of the Sith Empire.