Complex sentence: Then, Narcissus "tired from both his enthusiasm for hunting and from the heat" rests by a spring, and whilst drinking, "a new thirst grows inside him" and he is "captivated by the image of the beauty he has seen".
Split into simpler sentences: Then, Narcissus, tired from both his enthusiasm for hunting and from the heat, rests by a spring, and whilst drinking, a new thirst grows inside him. And he is "captivated by the image of the beauty he has seen".