Slith is the slang term for Slithzerkai, a lizardman-like race from the campaign world I started creating this adventure in (See Jeff Vogel's games, specifically Blades of Exile)
Sliths are huge, strong and smart, they lived in the underground world below the empire and the surface world, where they were encountered, fought and eventually befriended the people that the empire sent into exile in said underground world. Eventually, when there were enough (and powerful enough) people down in exile, some of who were sent there for felonies that their great-great-great grandparents commit (there was no way out, so people started families down there), the exiled peoples, as well as the Sliths, came up to the surface world and eventually learned to live peacefully with the imperials, integrating back into the empire as regular citizens.
More information about Sliths here:
https://encyclopedia.ermarian.net/wiki/Slithzerikai
In my basic info thread I wrote up stats for anyone who wanted to create a Slith character, in case anyone was interested.