The poetic device in the phrase "That Cruel Serpent" is a metaphor, since it doesn't refer to a serpent. It refers to a person who has such characteristics.
A metaphor is a figure of speech that depicts an object or action in a way that isn’t literally true, but helps explain an idea or make a comparison.
This poetic device states that one thing is another thing and it cannot be taken literally.
Metaphors are employed in poetry, literature, and anytime someone wants to add some color to their language