And Then There Was Silence by Blind Guardian is one of the best pieces of music ever written, a slow burn clocking in at 14 minutes I think? Surprised it's not already been mentioned here.

The Smoke of Many Fires by Be'lakor is death metal rather than power metal, but very similar in that it's a rather long song (~10 minutes) telling a story with an incredible climax, and is also one of the best pieces of music ever written.