I just thought up a device for a scifi or fantasy device in a culture that still has slavery. I'm thinking it might be a good one for Lyria's world. I'm calling it a Babel Shackle for now; put these on your slaves, and they won't be able to understand anything anyone says, except for the words of whoever has a Master Shackle on their person. Then the masters could communicate with their slaves, and slaves couldn't communicate with one another. It would work on all forms of language, because it would have an element of intent to it; if you intended to try to communicate, it would send a message about that intent to the other shackles, thus even coded winks or whatnot would be scrambled in the brain. Kind of the opposite of an Omnitongue talisman. (Omnitongue talismans allow the user to understand any and all languages. Well, almost all; some magical beings have languages that magically defy Omnitongue translation.) May even be useful to have some kind of magical AI built into it, to allow slaves to communicate work-related things to each other.
There may still be ways around it, but that would seriously hamper organized escape and/or rebellion attempts.