Here is a cool riff that starts in the first bar with a bluesy single line idea before developing from an interval (Am) into powerchords, octave slides and finishing with a turnaround of open string arpeggios! A lot of Opeth’s riffs will require you to develop a loose folk strumming technique rather than the more common thrash metal orientated rhythm posture, so move the wrist from the arm and hit the strings lightly.
You’ll notice at the end of bar 3 there is a chord that looks like a diagonal line. This chord is voiced R b5 R and implies a diminished sound which is common and effective in this style. I would recommend using alternate picking for the open arpeggios in the last bar.