Billboard has launched a lengthy piece profiling the history of Ozzfest with an oral history from Sharon and Ozzy Osbourne with other musicians detailing their memories playing the historic festival.

In 2004, a rare occurrence happened when Ozzy fell ill and Judas Priest’s Rob Halford stepped in to perform with Black Sabbath on August 26 in Camden, NJ. Halford recalls filling in for Ozzy.

Halford: That was an extraordinary day. I got a call from Sharon at my hotel, saying, “Ozzy’s not feeling too well. I don’t think he’s going to be able to perform. Would you help out?” I said, “Yeah, any time. When do you want me to do it?” She goes, “Tonight.” This is, like, about five hours before showtime.

Sharon Osbourne: I knew that Rob wouldn’t let us down. First of all, the friendship goes back so many years.

Ozzy Osbourne: Priest, my mates.

Halford: Sharon said, “You can do it. You can do it.” So I asked her, “Can you quickly send me the show?” They couriered a VHS tape to the hotel, and on the way from the hotel to the venue I put the VHS on in the bus and just sang along with the Sabbath performance. When I got there, I did the pre-show, had a shower, and then went out and did the Sabbath gig 20 minutes later.

Jane Holman (promotor liaison, Ozzfest): I mean, this is a once-in-a-lifetime event. I thought people should pay double for it.

Sharon Osbourne: It was fantastic for everybody involved, for the audience, for everyone there. Rob saved our ass, because we didn’t want to let the fans down.

Halford: I’ve made it known throughout my years as a metal singer that the two biggest bands in my life are Priest and Sabbath. There’s just an affinity between us. So it felt like not only the right thing for me to do, but the natural thing for me to do. And it was an absolute thrill.