Final answer:
Sibyl instructs Aeneas to obtain a golden bough, perform a sacrifice to the chthonic powers, and traverse the underworld towards Elysium, while encountering various landmarks and figures like Charon, doing so by initiating the journey with a sacred utterance for the esoteric path.
Step-by-step explanation:
The instructions Sibyl provides to Aeneas for entering the underworld include a series of steps and preparations grounded in Roman and Greek mythology. Firstly, Aeneas is required to find a sacred golden bough, which is necessary to gain access to the underworld.
Sibyl's guidance includes the signaling of a departure from her cave with the proclamation 'procul, o procul este, profani', a religious formula signifying the start of their journey to the underworld. This phrase implies the initiation into a sacred and exclusive experience, much like the Eleusinian Mysteries, suggesting that Aeneas is about to embark on an esoteric and divine path.
Upon approaching the entrance of the underworld, Aeneas must also perform a sacrifice to the chthonic powers, a common practice to appease the gods of the underworld. Once the Sibyl and Aeneas start their entry, they encounter various symbolic landmarks and figures, such as the ferryman Charon, who requires a small coin for passage, highlighting the currency of death in mythology.
The journey leads them to a fork where the right path leads to Elysium and the left to Tartarus, with the Sibyl advising to proceed toward Elysium. During their traversal, Aeneas is subjected to visions of future souls of his lineage, insight into Stoic cosmology, and the river Lethe, where souls drink to forget their previous lives. These complex and mystic instructions suggest Sibyl's role as both a guide and a ritual facilitator for Aeneas's journey into the underworld.