When the Foo Fighters announced a five-day residency on the Late Show With David Letterman as they’re getting ready to drop both an album and a TV show, it seemed likely that they’d debut five of the songs from that album. One night in, though, and that’s not what’s happening. Instead, they brought a country music star onstage to cover a metal classic. The night’s guest was Zac Brown, the Nashville superstar whose Zac Brown Band finds the unlikely border between mainstream country and jam-band festival fare. (Dave Grohl produced their The Grohl Songs, Vol. 1 EP.) Brown sat in as they covered Black Sabbath’s immortal 1970 destroyer “War Pigs,” and he showed an impressive level of muscle in his vocals, even if he did flub the “in the fields, the bodies burning” line. Grohl also sat for an interview with Letterman, talking about the ideas behind the Sonic Highways album and TV show and about his kids’ musical tastes. Watch the performance and the interview below.

The Sonic Highways TV show debuts 10/17 on HBO, and the album is out 11/10 on RCA. Don’t buy your kids Beatles records, dude, holy shit. Don’t be that guy.

[Photo by Stephen Lovekin/Getty.]