Detective Beauregard
Leviathan of Doggerland
I'm almost sure Mikael uses an English dictionary as he's writing the lyrics. He has fluent English, but some of his word choices are really interesting. Many words I hadn't heard of before listening to Opeth.
It's certainly possible, and I agree with the "words you hadn't heard before" statement. I find most of Opeth's lyrics to be very well-composed.