Complex sentence: Themistocles spares him instead, so he can warn Xerxes that the Greek forces are gathering at Salamis, and then visits Gorgo in Sparta while she is mourning Leonidas' death to ask for her help, but she is too overcome with grief.
Split into simpler sentences: Themistocles spares him instead, so he can warn Xerxes that the Greek forces are gathering at Salamis. He then visits Gorgo in Sparta while she is mourning Leonidas' death to ask for her help, but she is too overcome with grief.