As a non-english-talking-man I can say that I've heard a lot of new (for me) words in Opeth songs. I think it's because of Mike is also a non-english. Maybe he reads some dictionaries and select some interesting words that sound different? Reveries, perdition, advent, ethereal, cluster, benighted, affinity, dirge, apprentices, windowpane, hessian peel, heir apparent and so on. Of course it's not a ultra-rare words but it's not too easy to find another rock/metal songtitles with these words.
By the way, "Benighted" is one of the most beautiful words I've ever heard. Just like the song itself.