Complex sentence: Terrasson claimed he had translated this book from an ancient Greek work of fiction that was based on real events, although the book has been regarded as his own invention, being this inspired by ancient Greek sources that assumed Greek philosophy had derived from Egypt.
Split into simpler sentences: Terrasson claimed he had translated this book from an ancient Greek work of fiction that was based on real events. The book was actually his own invention, inspired by ancient Greek sources that assumed Greek philosophy had derived from Egypt.