Lavinia
by Ursula K. Le Guin
Lavinia, daughter of the king of the Latins of Laurentum, is sought after by neighbouring kings, but knows she is destined to marry a stranger. This is Aeneas from the Trojan War, who arrives with a large body of Trojans.
An agreement is made but then breaks down and there is war, which is won by the outnumbered Trojans. They found a new city called Lavinium, but Aeneas is killed after three years. Aeneas's elder son Ascanius founds Alba Longa and marries but fails to produce an heir. Lavinia removes her son Silvius from his control and he eventually becomes king of the Latins.
Rome already exists, but as a small settlement that plays no part in events.
Lavinia herself retreats from the world and at the end seems to have turned into an owl. She has all along regarded the world she lives in as unreal, a product of Virgil's imagination.