Complex sentence: By his wife Nysa of Cappadocia (who was the daughter of King Pharnaces I of Pontus) he had six children; but they were all, with the exception of one, killed by their mother, that she might obtain the government of the kingdom.
Split into simpler sentences: By his wife Nysa of Cappadocia (who was the daughter of King Pharnaces I of Pontus) he had six children. However, all bar one of the children were killed by their mother, so that she might obtain the government of the kingdom.