It might have been mentioned already, but a lot of Opeth riffs are played in the half-whole diminished scale. This scale alternates half steps and whole steps the whole way up, so in E it would be (E, F, G, Ab, Bb, B, C#, D, [E]). Along the E string in tab this would be like:
E---0--1--3--4--6--7--9--10--12--
Yes that's quite true ===> Just have a look at the most evil and dark sounding parts of Master's Apprentices...