Complex sentence: While trying to swim across the dangerous Euphrates, Abd ar-Rahman is said to have become separated from his brother Yahiya, who began swimming back towards the horsemen, possibly from fear of drowning.
Split into simpler sentences: While trying to swim across the dangerous Euphrates, Abd ar-Rahman is said to have become separated from his brother Yahiya. Yahiya began swimming back towards the horsemen, possibly from fear of drowning.