"Get thee back into the tempest and the Night's Plutonian shore!
On the pallid bust of Pallas just above my chamber door;
An allusion is a reference to an object or a circumstance from an unrelated context. This reference tends to be done indirectly, and usually without explanation, so that the reader can make the connection by himself. In this case, the "Plutonian shore" refers to the afterlife (guarded by Pluto), while the bust of Pallas refers to the goddess of wisdom, Athena.
