To be honest, you'll see it in various forms in the game--Isle of the Mist(s), (the) Isle of Mist(s), etc. Even Mist Isle, I think, but that one always struck me as an easy short form that people would be likely to use in conversation anyways. But admittedly those were just whoever wrote the lines in question mis-remembering exactly how it had been put forth in KQ6, and as usual, once the line's recorded, there it stays!