Answer:
I couldn't find which poem you are referring to, but I can help you define this word so you can compare if it fits with its use in the poem.
Explanation:
Something diabolical refers to something evil. It does not always have to be something strictly religious where the word refers to something related to "the devil." Sometimes it can be scary, tenebrous, or evil.
It is something excessively bad or wicked.
This definition should fit the poem you are talking about.
