Metamorphoses Book 12: Achilles and Cycnus
During the battle Achilles killed many Trojans, but when he came to Cycnus, he just couldn't kill the guy. As it turns out, Cycnus was the son of Neptune, and the sea-god was protecting him. When Achilles tried to kill the man with his bare hands, Cycnus was transformed to a swan. Both armies took a few days to rest and during the break Nestor told the Greeks about Caeneus.