A theme Tenkai Zuifeng, one of possible last bosses of the Neutral route.
Gotta say, "Prime Minister in Black Robes" is a joy to deliver, but a pain to survive through...
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The soundtrack comes from "Brilliant Pagoda or Haze Castle", 4th part of Len'en series created by Trick Nostalgie. The game is available for free here: http://www.freem.ne.jp/win/game/13429
Trick Nostalgie homepage: http://mugentrick.tubakurame.com/
The soundtrack has been extended to play for 20 minutes.