I'm going to treat this like a general metal discussion thread since I don't know where else to put this.
I was listening to Immortal's Diabolical Fullmoon Mysticism, and I found myself trying to think of what other metal it sounded most like. I realized that this album is more similar to something like Morbid Angel's BATS than it is to any black metal album. I would indeed almost call it a death metal album. Think about it; the riffing is often atonal and bizarre, the drums are relatively technical and occasionally even take the foreground, and the music is full of sudden spastic rythm - and tempo changes. All these elements are death metal trademarks.
Black metal qualities on the album are mostly taken from Under the Sign.. Bathory; lots of open strumming power chords where the lower finger shifts up or down while the upper finger stays in the same position (I don't know the terminology, fuck you). Obviously, the shrieking vocals and the many acoustic interludes are also similar to Bathory.
This obviously doesn't cover the whole sound, but if I absolutely had to combine two albums I would go with 60% BATS and 40% Under the Sign.
What two albums would you combine to get the closest to this - or other - album(s)?