Grotthu was a term referring to all slaves of the Red Sith, a species from the Stygian Caldera. They represented the lowliest caste of the Sith society. Originally, the Grotthu were of mixed species, although many of them were full-blooded Red Sith by 5000 BBY.
The Grotthu were a mixed-species group of slaves that were the lowest caste in the society of the Red Sith, a sentient race that resided within the Stygian Caldera. Known species found amongst the slaves of the ancient Sith included Herglics, Mrlssi, and Humans. Historians believed that many of these arrived as refugees who had fled unrest in the Tapani sector. However, around 5000 BBY, under the reign of the Dark Lord of the Sith Naga Sadow, many slaves were of clear Sith descent, with little to no phenotypical differences compared to the higher castes.
Occupying the lowest place in the pecking order that ran the Sith society, the Grotthu took care of all the laborious tasks. Even the full-blooded Red Sith among them had been turned into meek, servile creatures. During the funeral of a high-ranking Sith, several slaves were entombed with the deceased, as part of personal and cultural rituals.
Sorzus Syn, a Dark Jedi and one of the first Sith, remarked on the Grotthu in her chronicle of her time in the Sith Empire. She was pleased to see weak species subjugated by powerful beings such as herself.
- Book of Sith: Secrets from the Dark Side