Orestes, avenger of his father Agamemnon and killer of his mother Clytemnestra, turns up as a human sacrifice on the altar of his long-lost sister, a priestess of Diana. Welcome to the final, ultimately redemptive chapter of the saga of the bloody House of Atreus, retold in the decorous (some will say stilted), cool (some will say frigid) mode of the 18th century. Cecilia Bartoli has made a specialty of such material, and few know better than she how to bring it to thrilling life. (That said, we’re curious about Birgitte Christensen, Bartoli’s replacement for the last four performances.) —M.G.