Queen Klytämnestra has bad dreams, as well she may, for she butchered her husband. Her role in Strauss’s thunderous fresco consists of a single confrontation with her bloody-minded eponymous daughter (Nina Stemme), plus offstage screams when she meets her doom. Yet for many, Klytämnestra is the most haunting presence in the opera, painted in macabre colors that set the spine ashiver. In younger years, the Swedish soprano Karita Mattila excelled as Elektra’s sister Chrysothemis, whose heart is set on babies she will never bear. Now, Mattila graduates to the role of a mother whose children are the death of her. The music world is holding its breath. —M.G.