If you look at the lyrics to their other albums, this could be a continuation of stuff from Anthems to the Welkin & In the Nightside Eclipse; the fall of the empire of the narrator from those album's lyrics.
If you look at them as one story, In the Nightside Eclipse sets up an exposition for the story, Anthems tells of the rise of the narrator's empire, and Prometheus tells of his downfall and death.
If you look at the lyrics to their other albums, this could be a continuation of stuff from Anthems to the Welkin & In the Nightside Eclipse; the fall of the empire of the narrator from those album's lyrics.
If you look at them as one story, In the Nightside Eclipse sets up an exposition for the story, Anthems tells of the rise of the narrator's empire, and Prometheus tells of his downfall and death.