The Israeli-born Avner Dorman’s opera focuses on Richard Wagner’s clan and the composer’s legacy after his death, together with the family in-fighting presided over by his widow, Cosima, at the family home, Wahnfried. When the idea of the opera was first mooted, Wagner’s great-granddaughter Eva approved it, “as long as Cosima doesn’t come out of it very well”. She doesn’t, almost no one does, and certainly not the extraordinary and appalling figure of Houston Chamberlain. Chamberlain is so little known that he could be a figment of librettists Lutz Hübner and Sarah Nemitz’s imagination, yet the opera’s historical veracity is...