Baritone Yang Joon-mo, 52, describes Alberich, the dwarf in Richard Wagner’s opera “Das Rheingold,” as a role that places considerable demands on singers, requiring them to navigate a wide vocal range and master the forceful articulation characteristic of German. He will share the role with Samuel Youn, 55, a world-renowned Wagnerian bass-baritone who holds the prestigious German title of Kammersänger, in the Korea National Opera’s production of “Das Rheingold,” opening Thursday. “Das Rheingold” is the first opera in Wagner’s four-part “Der Ring des Nibelungen,” or “The Ring,” which took the composer 26 years to complete. After being mocked by the three Rhinemaidens while attempting to woo them, the dwarf Alberich renounces love, steals their gold and forges it into a ring. When Wotan, king of the gods, takes the ring from him, Alberich curses its owner with destruction. As the curse drives the tragedy that unfolds through the remaining three operas — “Die Walküre,” “Siegfried” and “Götterdämmerung” — Alberich is a pivotal character who set