The spell worked, but in a way that no one could ever have imagined.

While the players are imprisoned, the BBEG literally melts away into a puddle of goo over the course of several days. Let the PCs overhear rumors of this from their dungeon cell, but don't let them know -exactly- what is happening! Eventually, they're awakened by a riotous amount of noise. Weapons clashing on shields? Shouting? Looting!? What the hell is going on out there? The minions of the BBEG are literally looting the place because of the "pile of goo" that the BBEG has become, and because he is taking out his rage on all of them now. A slave or a guard unlocks the cell for the PCs, gives them their weapons and gear back, and flees the place.

The PCs now get to make their way back to the throne room to find the BBEG— who they still don't know what has happened to him —through half a horde of guards, with other guards helping them at times when the fights get tough, and then finally face off against the melting, dripping, ooze thing that the boss has become.