Complex sentence: These dynasties were founded by emancipated mamelukes, who had held high office at court and in camp under powerful amirs, and who, on their death, first became stadtholders for their descendants, and then usurped the throne of their masters.
Split into simpler sentences: These dynasties were founded by emancipated Mamluks, who had held high office at court and in camp under powerful amirs. When the amirs died, they first became stadtholders for the amirs' descendants, and then usurped the throne of their masters.