Slaves of Sargon
The Red Butchers are a well-organized group dedicated to freeing the slaves of Sargon. The group is comprised of multiple races with various motivations, but their activities are well-coordinated and they are much more than just a thorn in the side of the Minotaurs.
In recent years, the group seems to be more interested in toppling the Empire, than just freeing the slaves.
The Red Butchers have a few operatives inside Sargon cities and settlements who are actually slaves but secretly gather information and cause disruption where they can. Most of the Butchers reside in secret hideouts outside the settled areas (often in wooded areas).