Not to discourage any further answers but you'll probably find the FAQ on racism and slavery to be interesting. You should definitely check out these posts from the faq:

Europe had normal diplomatic relations with non-white nations before turning explosively racist to justify their actions against all others. What happened to cause this shift? by /u/victoryfanfare

Race in the Roman Empire by /u/cleopatra_philopater

And most of all check out What happened to the Roman system of slavery after the fall of the Roman Empire? Was the legal basis of 18th-19th century slavery derived from Roman law, or a completely separate system? by /u/textandtrowel