The poetic device shown in the bolded words "Assay'd to stifle guilt with sin" is oxymoron. An oxymoron is a figure of speech in which two contradictory or opposing words or ideas are combined to create a unique and thought-provoking expression. In this case, "stifle guilt with sin" combines the contradictory concepts of trying to suppress or alleviate guilt by committing sinful acts, creating a paradoxical and complex image in the reader's mind.