“If I rest, I rust.” At 78, Plácido Domingo still lives by this trusty watchword. But about a decade ago, after dominating more leading tenor roles for longer than anyone in operatic history, he began downshifting to the great baritone parts. Fifty years after his landmark Italian debut, he returns to the monumental Arena di Verona to showcase three of these recent acquisitions in full-act excerpts from three Verdi masterpieces, including Macbeth. An evening for the history books. —M.G.