“Why are the senators sitting there without legislating?” asks C. P. Cavafy in his poem “Waiting for the Barbarians.” It continues, “Because the barbarians are coming today. / What’s the point of senators making laws now? / Once the barbarians are here, they’ll do the legislating.” Under the Gothic arches of Saint Thomas Church Fifth Avenue, Death of Classical and The Alexander S. Onassis Benefit Foundation present a one-night-only musical exploration of “love, loss, lust, and longing” inspired by Cavafy’s poetry. With new works by musical artists such as Rufus Wainwright and Missy Mazzoli, Laurie Anderson, Nico Muhly, and more, it sounds like a happening. Premium seats are sold out, but general admission is still available. —Laura Jacobs