Complex sentence: The Holy Roman Empire is perhaps the best-known example of an elective monarchy (although it eventually became unofficially hereditary); after the 15th century, the emperor was elected from within the House of Habsburg by a small council of nobles called prince-electors.
Split into simpler sentences: The Holy Roman Empire is perhaps the best-known example of an elective monarchy. During that period, the emperor was elected from within the House of Habsburg by a small council of nobles called prince-electors.