The prog-rock pioneers embraced extravagance: odd instruments and fantastical lyrics, complex compositions and abstruse concept albums, flashy solos and flashier live shows. Concertgoers could savor a new electronic keyboard called a Mellotron, a singer dressed as a batlike alien commander, an allusion to a John Keats poem, and a philosophical allegory about humankind’s demise—all in a single song (“Watcher of the Skies,” by Genesis).