Metamorphoses Book 13: Hecuba, Polyxena, and Polydorus
Priam had sent his youngest son, Polydorus, to live in Thrace with Polymestor. When Polymestor learned of Troy's fall, he killed Polydorus and kept the gold that Priam had given the boy. He threw the body into the ocean.
Agamemnon and the Greeks stopped in Thrace during a storm and Achilles' ghost appeared to them and demanded that Polyxena, Priam's daughter, be sacrificed in his honor. The girl went to her death bravely, and Hecuba mourned her daughter.
"'He who destroyed your many brothers, / He destroyed you, Achilles, doom of Troy / And my bereaver. But when Paris' arrows / And Phoebus' felled him, now for sure, I said, / We need not fear Achilles: now again / I had to fear him: in the sepulchre / His ashes raged against our race; entombed, / We felt him as our foe. For him I bore / My children! Mighty Ilium lies low. / In tragedy our nation's ruin reached / Its end; but end it has. For me alone / Troy lives; my woes stream on.'" Book 13 -- Hecuba, Polyxena, and Polydorus, line 501-10
Achilles had killed all of her children except Polydorus. His life was her salvation until she found his body on the shore. Plotting her revenge against Polymestor, she tricked him into meeting her under the guise of giving him more gold for Polydorus. When the king of Thrace met with her, she clawed his eyes out. The Thracians attacked her for injuring their king, and she was transformed into a dog.