It’s not often that a pair of technically proficient jazz musicians pull off a life-altering show that is simultaneously the stuff of nightmares. Welcome to Clown Core-chella. Then the madness began.

When festivalgoers packed into the Sonora tent Friday for the venue's last set of the day, avant-garde jazz and grindcore duo Clown Core, they had some idea of what they were getting, but it was even ...