The lines Oliver quotes are from
Hamlet, from the scene in which the ghost of the dead king describes his afterlife to Hamlet. The “prison-house” that the ghost alludes to is purgatory. Oliver is using the quotation literally—he really is in a prison—but also to suggest the mental punishment of his 10 years of solitude. His agreement to Colborne’s proposition on his own terms suggests that Oliver wants to share the burden of his story, but he’s wary of his words being used against him. Thus, the novel’s opening sequence immediately establishes the power (and danger) of words.