And, of course, even barring slavery, there are a number of conditions that fail to quality as "slavery" only by the thinnest of technicalities. Indentured servitude, debt bondage, serfdom, etc. are all "slavelike" conditions that persisted even in areas where slavery itself was uncommon or illegal.
In historical times, it's sometimes staggering to realize how many people lived in slavery. 25-35% of the Roman population were slaves, according to some estimates. In the early middle ages, shortly after the Norman conquest, 10% of Englishmen were slaves. At roughly the same time, as much as half of the population of lower Iraq may have been black slaves from Zanj (and when the Zanj slave revolts happened, the Islamic caliphates turned to the Christian southest for its slave; taking millions of slaves from the Balkans.) Over a million Europeans were captured in the heyday of the Barbary pirates and enslaved. In the west African polities of the last few hundred years, including Ghana, Mali, Segou, and Songhai as many as a third of the population are estimated to have been slaves. Slavery was still legal and widely practiced in the Ottoman Empire and China as late as about 1910 or so, and illegal or not, persisted beyond that. The list goes on and on.
The philosophical idea, originally popularized in America and Western Europe of the native dignity of the individual, inalienable rights, and the notion that all men were created equal (or at least should be empowered by equal opportunity) is one that we take for granted today, and I'm certainly glad of that (not least because I'm a direct beneficiary of such philosophies) but it shouldn't be forgotten that it's also an idea that would be very foreign to most people who have lived on the earth throughout it's history. Not only foreign, but unpalatable and even dangerous, even to slaves themselves, in many instances. Not that slaves were always enamored of their condition; the very frequent reports of miserable conditions, slave revolts, and more certainly would prove any such notion as absurd--but that doesn't mean that slaves saw the institution of slavery itself as the problem; rather their own circumstances therein was what they were unhappy about.
Anyway, why is it that I'm saying this? Slavery exists in the Dark•Heritage setting, that's why. Not only does it exist, it's widespread and not a single society or culture doesn't practice some form of it. The most liberal and enlightened approach would be that it's illegal to enslave it's own citizens (or the citizens of allied states) except in the case of debt slaves, sentenced criminals, prisoners of war, etc. Many societies don't even practice that, of course--they happily pursue slave-taking raids and expeditions, and the slave trade and slave labor is a huge portion of what makes their economy prosperous.
I've often described Dark•Heritage as dark fantasy. That's a little bit of a nebulous term, though. What exactly does it mean? Many definitions that I've seen (including Wikipedia's) say that it's a juxtoposition of the horror and fantasy genres. However, I think there's a lot more to it than that--in other ways it can also deviate from the buccolic, romanticized Medievalism that was so prevalent early in the genre and which persists stubbornly today. In Dark•Heritage, there's no such thing as a philosophy that lauds the dignity of the individual. Human life is cheap and relatively valueless. Indeed, many people are considered less useful or worthy of preservation than that of an individual bison or horse. Slavery drives home the theme that Dark•Heritage is not a happy-go-lucky high fantasy; it is a gritty, dangerous, sword & sorcery and dark fantasy setting where life is fast and cheap, and there are few (if any) to mourn those who fail for whatever reason to make the most out of their lives that they can.