Book 20 

Zeus worries that Achilles’s rage might be powerful enough to alter fate itself, so he calls the gods together in Olympus and orders them to enter the battle on whichever side they choose.

Aeneas, spurred on by Apollo, challenges Achilles; however, Poseidon rushes in and spirits Aeneas away from the field. Despite this, Achilles still kills Polydoros, Hector's brother. Before Achilles can kill more important Trojans, Apollo wraps Hector in a thick mist, disguising him from Achilles’s rage. 

Book 21

Achilles's onslaught forces the Trojans into retreat, with one group running toward the city and another toward the river Xanthos. The enraged Achaean leaps into the river after them and kills a great number of Trojans, sparing twelve young men in order to fulfill his promise to Patroclus.

However, Xanthos is angered by the destruction occurring in his waters and gives courage to Asteropaios, who challenges Achilles in single combat but ultimately loses. Poseidon and Athena come to reassure Achilles that he will not die at the hands of the river; Hera and Hephaestus rescue him from it using fire.

Meanwhile, the gods fight each other: Apollo refuses a challenge from Poseidon out of respect for his uncle; Hera takes on Artemis who runs away crying after her father, Zeus, intervenes.

Priam sees Trojans fleeing toward city walls, which are opened up for them while Agenor faces off with Achilles to cover their retreat. Before Achilles can strike a killing blow, Apollo snatches Agenor away to safety. The god then tricks Achilles into chasing him down a plain, so the Trojans might escape safely inside the city walls.