Complex sentence: When they reached what is now Angola in the sixteenth century, they encountered the Khoisan, Bushmen and other groups considerably less technologically advanced than themselves, whom they easily dominated with their superior knowledge of metal-working, ceramics and agriculture.
Split into simpler sentences: When they reached what is now Angola they encountered the Bushmen and other groups. They reached what is now Angola in the sixteenth century, where they encountered the Khoisan, Bushmen and other groups considerably less technologically advanced than themselves, whom they easily dominated with their superior knowledge of metal-working, ceramics and agriculture.