Slaves to Darkness are the epitome of the human side of Chaos. Of course there are demons as well.....Here is a little lore provided by GW today.
via the Warhammer Community
https://www.warhammer-community.com/2019/12/05/slaves-to-darkness-the-loregw-homepage-post-1/
The Slaves to Darkness in the Mortal Realms
The Mortal Realms have long struggled under the yoke of Chaos, and as such, the Slaves to Darkness, in all their myriad forms, are their most widespread human inhabitants.
Beyond the walls of Sigmar’s fledgling civilisations are countless fallen empires and barbarous tribes, as diverse as the realms themselves. While the Stormcast Eternals and the forces of Order see themselves as liberators, the Slaves to Darkness see them as cravens who abandoned the people of the Mortal Realms at the hour of their greatest need.
Where They Are Now
Even in the Age of Sigmar, the Slaves to Darkness maintain an iron grip on the Mortal Realms, particularly Aqshy and Ghur. Battletome: Slaves to Darkness explores these warriors in more detail than ever before, fully cementing them as a major player in the Age of Sigmar and exploring their varied (and sometimes even noble!) motivations.
When writing the battletome, the Warhammer Studio got in touch with a man who knows just how to make you fall in love with the bad guys – (in)famous Black Library author Aaron Dembski-Bowden. Aaron makes his Warhammer Age of Sigmar writing debut with four short stories in Battletome: Slaves to Darkness.
Follow the link above to see more from Aaron.