Final answer:
The Golden Bough is the key allowing Aeneas to safely enter the Underworld in Virgil's Aeneid, symbolizing his fated journey.
Step-by-step explanation:
The key that allows Aeneas to enter the Underworld in Virgil's Aeneid is the Golden Bough. Aeneas acquires this bough with the help of his guide, the Sibyl of Cumae. The bough is a token of passage for Aeneas to travel safely through the Underworld. Its importance is rooted in various mythological sources, including Orphic, Eleusinian, and Hellenistic-Jewish traditions.
Ultimately, it symbolizes Aeneas' destined path to the Underworld, which is integral to his mission to establish the future Roman race.