Complex sentence: Alcyoneus was one of the traditional opponents of the hero Heracles, who was (along with Porphyrion) one of the greatest of the Giants, the offspring of Gaia and Uranus.
Split into simpler sentences: Alcyoneus was one of the traditional opponents of the hero Heracles. He was (along with Porphyrion) one of the greatest of the Giants, the offspring of Gaia and Uranus.