Hallowed are the children of the Void
—Common phrase among the Thrall Caste
The Thrall Caste was one of the three main castes of Charon society, alongside the Savant Caste and the Zealot Caste. The second largest of the three castes, the Thrall Caste made up the slaves, servants, scientists and stewards of the Charon Dominion. Subordinate to the other castes of Charon society, the Thrall Caste invests little in philosophy outside of the necessity inscribed unto them for exclusive devotion to the "children of the Void", being the Savant Caste.
While effectively servants to the Zealot Caste as well, the Thrall Caste predominantly serves the "Master Caste" in any measure possible to further their goals and agendas, whether through manual labor, engineering work, scientific research and development, or in any other capacity. The Thrall Caste was comprised of species dominated during the Oblivion Crusades that, while deemed unfit to serve as indoctrinated soldiers in the Zealot Caste and failed to produce Voidborn for the Savant Caste, possessed attributes that made them ideal for supporting the other castes nonetheless.
Some species among the Thrall Caste that displayed consistent levels of high cerebral aptitudes, for instance, might be in servitude to the Dominion through their intellectual prowess in developing weapons, defenses or war machines. Others species might have been inducted into the Dominion to make use of their administrative capacity, assigning them to be curators, stewards or keepers. The Ixaya, arguably the most iconic species of the Thrall Caste, much as Humans and Vrathites are for the Savant and Zealot Castes respectively, provide the Dominion with a huge resource of slave laboring.
As noted by Savelle Ulesari, the difference between a species of the Thrall Caste and one extinct after coming into contact with the Dominion, is that the Savants saw either a quality or potential within the species that might further their efforts toward galactic extinction. Any species among the Thrall Caste that is eventually found wanting, or becomes overshadowed by the aptitude of a newly subjugated species, is "purified" into the Void, a ritualistic form of genocide.
Unlike the Savant Caste and Zealot Caste, the Thrall Caste had no hierarchy or echelon to speak of, save for the position of Chamberlain, the personal slave of the Prophet of the Void. While technically no rank or authority came with the position, the Chamberlain was often treated and considered the most senior of the Thralls. Xaga Terron once mockingly referred to Elac Qur, the Chamberlain of the Harbinger during the Extant War, Krovo Scrios, as the "king of slaves".