It's clichéd to describe a concert as a once-in-a-lifetime experience, but in the case of D'Angelo's visit to the Mainroom last summer, such lofty rhetoric couldn't be more appropriate. This was the stuff of legend. The troubled R&B star had seemed all but lost to the ravages of drug abuse and personal demons in the decade-plus since his last appearance in the Twin Cities. His return betrayed every bit of the considerable effort it took for him to get up on stage. Even the beaming irreverence of D'Angelo's funky-drummer sidekick, the Roots' Questlove, couldn't dull the emotional sting of deep cuts from Sly Stone and the Ohio Players, marinated in a hazy, burnt-out malaise of self-reflection and boiled down to the grimiest, most viscous soul imaginable. But then he brought it back from the brink with the most over-the-top double encore in recent memory, topped off with a tear-the-house down, 20-minute rendition of the Family's "Mutiny" that may as well have been a glimpse of Prince on home turf in his heyday. The night's exorcism took its toll, though: In the days after the First Avenue show, D'Angelo canceled his subsequent appearances, including a return visit to Mainroom scheduled for later in the summer. Once in a lifetime, indeed.