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That musical was pure kino I don't care what anyone says.
Maintaining Shakespearean English the entire time, having the stage be literally a fucking book and them literally sinking into the pages when it changed, that final Dark Souls-ass boss music version of the Promise theme when the reveal happened, all of the particular songs and interpretive dance numbers and the sheer amount of yuri ship faggotry in there - all of it was cool as fuck and not what I was expecting.
I 100% thought it'd just be a thinly disguised standard 3D live with brief skits inbetween barely justifying it being a 'musical' but it was so much more than that even if the ending was kind of weird and hard to fathom because it was presented purely by dance and song.
Here's my interpretation of the ending and why it ends on such a dark boss-music-esque note and what the title means:
it's actually not a happy ending but is instead a 'the bad guy wins' downer ending - if you think about the opening narration describing the 'Dark Lord' by ERB you notice she descibes them as having a multifaceted and almost contradictory personality. The throne room at the end had 5 thrones and they realized nobody was actually there in the 'Dark Lord's castle the entire time and then took the seats and looked ominous. Here's what I think; the alleged Dark Lord never actually existed as a single entity and was actually collectively all five of them, THEY were the Dark Lord all along and were destined to take this role due to their own vices which matched the different aspects of the Dark Lord's personality (e.g. ERB says the Dark Lord was manipulative which matched Faelyn, lustful which matched Kora etc.)
The ending is not actually them defeating the Dark Lord's minions on their quest and then reclaiming their thrones to instill goodness in the realm but instead them realizing they collectively ARE the Dark Lord, seating the thrones and plunging the realm into a reign of terror by all five. They ironically all got their wishes by taking power by force over the land e.g. Hexa DID in fact become renowned in all the realm - because she conquered it and became feared. The reason why they were able to become the Dark Lord is and come to this revelation is actually because of the fact that their own vices drew them apart and darkened their hearts as seen in the conflict over Namu's bag before the climatic song numbers. By the time they arrived at the throne room they were all embittered and could only unite through their shared understanding that they were the villains all along.
It's called 'The Broken Promise' because of both the fact that they had to break their bonds as a party to become villains and fulfill their dark destiny and also because the promise to defeat the Dark Lord was broken and could never be fulfilled - because they could not defeat themselves.