I think aeternus is a better choice than perpetuus. The English derivatives in this case are good guides: aeternus is “eternal,” and perpertuus “perpetual” is continuous and uninterrupted but the focus is on the unbrokenness rather than being neverending.
I don’t think we’ll hear from the OP again, though. Perhaps (s)he has already gone in noctis aeternae tenebras.
[Re: syntax: is that right, or is it better to say in tenebras noctis aeternae? I know prep. + gen. + noun is fine, but I’m not sure what typically happens when the genitive modifier in a prepositional phrase has its own adjective.]