If you're a fan of The Cardigans' angelic cover of Black Sabbath's "Iron Man," or of English singer-songwriter Ed Sheeran, Jimmy Fallon has a bit for you. "That's right, pop music's cherub-faced troubadour sings 25 of the darkest, most skull-crushing songs ever written," Fallon, in character, said in a faux late-night commercial on Monday's Tonight Show.