"It is a new song that's kind of Helter Skelter meets Scentless Apprentice. It rocks with a heavy groove. St. Paul is powerful. @Pontifex," he wrote.
Other performers include Billy Joel, Alicia Keys, Bon Jovi, Bruce Springsteen, Eddie Vedder, Roger Waters, Kanye West and The Who. The concert will be live-streamed on PEOPLE.com as well as 121212concert.org.
Whether you think it's a great idea or not, here's what fans have to say on social media about McCartney fronting a Nirvana reunion.