A Slave is a person who has been forced into indentured servitude by the nation they reside within. This can happen in many ways, including, but not limited to:
Punishment for violation of (a) law(s).
Speaking out against the ruling Monarchy.
Resisting conquest by the enslaving Kingdom's armies.
Being kidnapped by skirmishers.
Engaging in practices frowned upon by the aristocracy.
And so on. This is a brief example, not a comprehensive list of possible backgrounds for characters.
Slaves are a source of cheap labour that usually lasts between twenty and fourty years. To that end, they are in no ways cheap. Slave prices typically range between 50 and 400 Gold, with deviations for particularily sickly, mentally/physically outstanding, or otherwise notable specimens. Price is affected by health, efficiency, age, status before enslavement, and physical appearance. Enslaved Kings usually go for between 1000 and 4000 Gold. When purchased in bulk, these factors that affect the price of individual slaves are commonly overlooked. Slaves sold in bulk are, more often than not, cheaper than slaves sold individually.
Clever Slavers tend to mix Slaves of different ethnicities and languages as to minimize communication between the workers, thus lowering chances of escape. Slaves that spoke a foreign language were commonly taught commands and phrases in the language of the host country.
** Editor's note: Please do not free the Slaves at MRP.