Complex sentence: The book reveals the creature was once Antoine Domoduro, one of the island's original inhabitants who became stranded on it with his wife and daughter during the atomic bomb tests, which killed his family and mutated him.
Split into simpler sentences: The book reveals the creature was once Antoine Domoduro, one of the island's original inhabitants. He became stranded there with his wife and daughter during atomic bomb tests, which killed his family and mutated him.