The True Sith was a faction of the Sith which ventured from Korriban into the Unknown Regions after the death of King Adas and the Sith civil wars that took place soon after. These True Sith departed Korriban around 27,500 BBY, fleeing to the Unknown Regions. Fearing that the Rakata would be able to trace them through the powerful Force-sensitivity that the Sith possessed and attempt to enslave them again, the leaders of the True Sith utilized their archaic Sith Alchemy to render themselves and their fellow Sith invisible to the Force at a basic level. As the True Sith continued to breed, this skill became a natural and genetic trait amongst every Sith, rendering them completely invisible to the Force, though able still to interact with it in any other way.
As the True Sith continued to propagate in the Unknown Regions, they developed an unparalleled mastery of Sith Alchemy, and over time began to use it more and more, to the exclusion of all other technology and Force skills. Such was their mastery of alchemy that they were able to enhance the attributes of living organisms, turning them into deadly weapons or useful devices, and even learned to construct starships of Force-reinforced corals. As their skill with alchemy and transforming living organisms progressed, the True Sith developed an ever greater hatred of non-organic technology, it being dead to the Force and unable to be bent by their alchemy.
Through their vast use of alchemy and the legends of their ancestors on Korriban, the True Sith came to embrace the dark side of the Force as a binding force within the galaxy, a semi-sentient omniscient power. Their worship of the dark side did not deify it, but rather sought to understand how best to use it to further enhance their own inner power. At some point, the true Sith learned that the main fuels of the dark side were fear, anger and pain, all emotions that could empower the possessor with the dark side and greatly enhance their alchemy and sorcery. In order to embrace these emotions, the True Sith began to offer up parts of their own bodies in painful and terrifying mutilation rituals, only to replace them with Force-imbued alchemical prostheses that caused further pain and continued to reinforce the flow of the dark side through each individual Sith.
There are very few artifacts of the True Sith within the currently Known Galaxy, other than the Trayus Academy on Malachor V. The True Sith were deemed to be a significant threat to the galaxy by Revan from what he learned of them at the Trayus Academy, and his whole Sith War was in order to establish a stronger galaxy to fight these True Sith. Supreme Commander Thrawn and his Chiss associates stumbled across the True Sith during the Galactic Clone Wars, and the threat posed by the True Sith was enough to draw Thrawn into the galactic war, in order to gain access to the military technology of the Galactic Republic. Since the end of the Clone Wars the Empire of the Hand has been engaged in brush-fire conflict with the True Sith, attempting to prevent them from gaining a foothold in the galaxy at large.