Article continues below advertisementElvis Presley didn't know about his ex-wife Priscilla's "Burning Love" with Robert Kardashian Sr.In her second memoir, Softly, As I Leave You: Life After Elvis, Priscilla shockingly revealed she kept her and the Kardashian patriarch's relationship a secret from the King of Rock and Roll, whom she divorced two years before she and Robert started dating in 1975.Priscilla never came forward about her next romance before Elvis devastatingly died of a heart attack in 1977 at age 42.