Leon Russell watched Elton John make his US debut from the front of West Hollywood’s famed Troubadour nightclub in the summer of 1970. Seeing his idol a few feet away blew Elton’s mind, but not his cool — that Troubadour gig is one of rock’s most legendary star-making shows.

Four decades later, the two piano men unite for a mutually autumnal career highlight. T Bone Burnett replaces John’s band with heavy hitters — guitarist Marc Ribot, fellow star producer Don Was on bass, steel guitar maestro Robert Randolph, Beatle pal drummer Jim Keltner and Southern soul mainstay Booker T. Jones on organ — and the eerie results take Elton way beyond his Vegas comfort zone.

Russell sets a somber, yet darkly humorous tone with “If It Wasn’t for Bad,” but Elton and Bernie Taupin match his mettle with much of the rest, including the Civil War-themed “Gone to Shiloh” with Elton, Leon and Neil Young each singing a verse. The Ray Charles influence throughout is undeniable: The Union is akin to Daptone Records’ vintage R&B recreations, but with Charles replacing James Brown as the guiding artistic light.