something about prisoner of war being forced to work or fight for the country that held them captive. captive is very skilled and captures the attention of the country who capture him or her.

Threads of Time

I agree with vnvnvn on Threads of Time. It only happens around the middle though. Another would be Ares maybe? One of the main characters used to be a war slave. But the story doesn't really center around that guy. Ubel Blatt?? Later, like much later in the story there will be a guy(really skilled) who is a slave knight, appears as the Hero's rival/enemy. Don't know if these will help much.