Testament legend Chuck Billy recently lent his voice to a special cover of Motorhead’s “Ace of Spades,” but this wasn’t done in the studio or in front of thousands of spinning thrash fans; it was at a family birthday party.

While smoking a cigar, Billy joined a rockabilly / country group called Outlaw Cash Prophets onstage during a party in the Bay Area. The “Ace of Spades” rendition was done in the style of Johnny Cash. Outlaw Cash Prophets write that they’re straight outta Folsom, Calif., where Johnny Cash famously performed at the local prison in 1968.

It may not have been a traditional performance of “Ace of Spades,” but Billy seemed plenty comfortable onstage with the band and knew the lyrics perfectly. The video’s description reads, “Chuck Billy of Testament sings Motörhead's ‘Ace Of Spades’ with Bay Area band, the Outlaw Cash Prophets at a family birthday party in the Bay Area in Rockabilly, Johnny Cash infused cover that would make Lemmy proud and probably include in the Head Cat sets, if he were still with us m/“

Check out the performance above. Testament are expected to release their 11th studio album, The Brotherhood of the Snake, later this year. It’ll serve as Steve DiGiorgio’s first studio recording with Testament since 2001’s First Strike Still Deadly.

See Testament on the Top 50 Hard Rock + Metal Debut Albums

See Where Testament Ranks on the 10 Greatest Thrash Bands