Final answer:
Phaethon's downfall was due to his hubris and inability to control the sun chariot, leading to Zeus striking him down to save the Earth.
Step-by-step explanation:
Phaethon's downfall was caused by his unchecked ego and desire to prove his divine heritage. He convinced his father, the Sun god Helios, to let him drive the sun chariot across the sky. Unable to control the powerful horses, Phaethon set the Earth ablaze, threatening all life. The god Zeus intervened by striking him down with a thunderbolt to save the world, leading to Phaethon's demise. This myth emphasizes the dangers of hubris and the consequences of overstepping one's limits.