Among the most magical of all opera scores, Pelléas and Mélisande, Claude Debussy's only completed opera, centers on two fallen lovers who marry and then return to a dark ancestral castle. Yuval Sharon (The Cunning Little Vixen) directs this made-for-Cleveland production. The performance takes place at 7:30 tonight at Severance Hall, where it repeats at the same time on Saturday. (Niesel)