BREGENZ, (Austria) - While the opening night of the annual Bregenz Festival on Wednesday was one for the masses with a visually stunning staging of Giacomo Puccini's "Tosca", opera connoisseurs were in for a treat on Thursday with a rarely performed operatic gem from the 1930s. "Karl V", a 12-tone "drama with music" by exiled Austrian composer Ernst Krenek (1900-1991), has only ever been performed a handful of times since it was premiered in 1938 " in Essen, Duesseldorf, Zurich, Munich, Vienna and Salzburg. The Bregenz Festival has made it its task to unearth and stage more difficult and lesser-known works, alongside the usual operatic blockbusters it puts on for the wider public, says artistic director David Pountney. "Karl V" " reputedly the first evening-length opera using the twelve-tone technique developed by Arnold Schoenberg (1874-1951) " tells the life story of the Holy Roman Emperor, Charles V, in the form of flashbacks. On his deathbed, the emperor makes his confession to a young monk, explaining his actions, his motives in an attempt to obtain absolution. In a stroke of genius, German actor-director Uwe Eric Laufenberg sets the entire opera as a history lesson, complete with school classroom and the emperor as schoolteacher and the monks as his pupils. The idea is as simple as it is brilliant and Laufenberg uses it as a vehicle to explore not only 16th century Europe, but also Europe in the 1930s with the rise of national socialism and fascism. Laufenberg has also assembled a top-notch cast, first and foremost the consummate German baritone Dietrich Henschel in the title role. Henschel, whose sinewy frame belies the warmth and nobility of his voice, has made a name for himself in demanding roles of 20th century opera. The emperor, who is on stage almost the entire three hours, is no exception and he mastered the extremely taxing part with astonishing skill and ease, utterly credible in the role. German-Spanish soprano Nicola Beller-Carbone was also outstanding as the emperor's sister, Eleonore. German conductor Lothar Koenigs was a careful and attentive guide through the difficult score with superb playing by the festival's house orchestra, the Vienna Symphony Orchestra. Krenek forms the main focus of this year's Bregenz Festival, which runs into August 23. "Karl V" is being performed four times in all. Another stagework, the satire with music "Kehraus um St. Stephan", is also being staged in Bregenz's Corn Market Theatre and there will be performances of a number of his orchestral works, including his First and Second Violin Concertos. Next year, the Bregenz Festival will stage Verdi's "Aida" on the stage lake, while another operatic rarity, "King Roger" by Karol Szymanowski (1882-1937) in a new staging by David Pountney, will be resurrected in the Festspielhaus theatre.