lines that don't scan right: "And my own wretchedness too deep" "I see his instruments of despair" "I've few years left to perform these rites" "so break Her irons, cold shackles of Death" "shall answer my anguished cries this night?" have one too many syllables; "requests an audience with Demons!" is just wrenched; "And for this I leave you now" has one too few.
"long the coming" appears to be a rhythmic necessity but it doesn't make grammatical sense.
good poem...powerful story...the end of the last stanza is incredible.

One word: Awesome!
Two words: Magically Sweet
I love the poem! Write more poems like this. I am really interested in supernatural stuff like this. Are you? I wrote one supernatural story, called Dinner with Satan. You may enjoy it.