“Often, we see the wrong side of destinies,” says the blind king in Debussy’s doggedly obscurantist faux-medieval tragedy of forbidden love. The dialogue is lifted verbatim from the play by Maurice Maeterlinck, arguably the spaciest laureate in the history of the Nobel Prize for Literature. The work is virgin territory for Valery Gergiev and the Mariinsky Theatre, which unveils a new production not in one of the company’s two fully equipped opera houses but in its phenomenal (still) new concert hall, in deference, perhaps, to its preternaturally revealing acoustics—just the ticket for Debussy’s billowing tapestries of sound. —M.G.