As far as Very Definitely Final Dungeons go, the Black Omen is unsurpassed in magnificence. It has everything going for it: critical importance to the story, arch-villain at the end of it (the deranged Queen Zeal), the feeling of impending finality, grand architecture, a tinge of weirdness, and above all, the inescapable feeling that this is some major, major shit going on right now.
Oh, and of course the ass-kicking music plays into that a lot.
Oh, and did I mention that if you really want to, you can run this dungeon three times? In the same game. Hey, it's time travel after all.
What was, acceptably translated, called the Black Dream originally was renamed the Black Omen in the U.S. version, for reasons completely mysterious to me. Maybe they thought they'd run afoul of Martin Luther King Jr.'s legacy, I don't know.