If someone was taken prisoner by the Thule, is there any chance that they would be marked by them with a brand or scar or something? I’m thinking of something similar to the ‘B’ that Spartacus has burned into his arm in the TV show. Is this a thing at all in the empire?
I common rune AFAIK that has been used is Hirmok the rune of Dominion, also called the Rune of Tyrants.
Though this would only really be a northern thing as Thule and Jotun use Runes (though latter don’t in their mind keep slaves, rather Thralls).
There are several player characters wandering around with the word ‘SLAVE’ carved into their foreheads as a result of being captured by the Druj. An ‘S’ would probably work just as well.