One of the hidden influences on death metal, along with classical and progressive rock, was the wave of inventive ambient and electronica that came out in the 1970s.
In particular, this music like death metal, was highly structured in that verse-chorus structures would turn it into droning tedium. Thus it invented the narrative structures later used by death metal, the “riff gluing” as explained by Asphyx.
Tags: Ambient, influences
Narrative song structures have been used by composers for centuries, if not millenia. Death Metal might just as easily have taken that influence from Western Classical or Folk music (and probably did, depending on the band). I always saw the electronica/ambient influence as affecting Black Metal more than Death Metal.