Complex sentence: The only later exilarch whose name is recorded is Hezekiah, an exilarch who also became "gaon" in 1038, but fell from power in 1040, both the last exilarch and the last "gaon".
Split into simpler sentences: The only later exilarch whose name is recorded is Hezekiah, an exilarch who also became "gaon" in 1038, but fell from power in 1040. He was the last exilarch and the last "gaon".