Metallica and King Diamond go way back. The Cali thrash icons famously recorded an 11-minute medley of Mercyful Fate songs for their 1998 album, Garage, Inc, and over the years, King Diamond has made special guest appearances when Metallica perform the suite live. However, what must have been one of their first moments of onstage camaraderie took place at a San Francisco King Diamond show way back in 1986, and it was quite a sight to behold.
As seen in the immortalized video of the special moment, King Diamond took time in the middle of his set to invite "a couple of guys called Lars, Kirk and James" up on the stage. While he's charismatically introducing the budding stars to the rowdy crowd of club-goers, the three Metallica members in question dash out to join Diamond while looking visibly intoxicated — and the reason why becomes clear soon enough.
"I'd like to say to James, from us up here and from everyone inside here, have a fucking happy birthday," Diamond shouts before he and his bandmates dive into a lovably sloppy rendition of "Happy Birthday." Shortly afterwards, Diamond makes a pun out of his song, "No Presents for Christmas," by saying "No Presents for James," and the band rip into a seasonally-appropriate version of their Christmas single from the year before.
Metallica spend the rest of the song intermittently sharing the mic with Diamond and headbanging like idiots while people in the crowd try to surf up to get in on the action. It looks like an absolute riot, and you can watch it unfold above via YouTube.