Complex sentence: He was a famous tamer of horses and king of Etruria, known for being one "whom no one can fell by fire or steel" (Mandelbaum, VII.911-912); perhaps because he is a son of Neptune.
Split into simpler sentences: He was a famous tamer of horses and king of Etruria. He could not be "fell (ed) by fire or steel" (Mandelbaum, VII.910-914); perhaps because he is a son of Neptune.