BABYMETAL (@babymetal_official on Instagram)

Babymetal have been opening for Red Hot Chili Peppers for their U.K. tour all month, and the metal group ended their trek in a very special way. RHCP drummer Chad Smith joined them onstage at London's O2 arena last night (Dec. 18) to cover some Judas Priest tunes.

Smith, who dressed up and rocked white face paint Kami Band-style, replaced drummer Hideki Aoyama to perform a killer solo. The artist then launched into performances of two Judas Priest classics: 1980's "Breaking the Law" and 1990's "Painkiller."

If that wasn't cool enough, Smith also sang "Happy Birthday" to Su-Metal (who turned 20 yesterday) and rolled out a birthday cake! See all the fan footage below via NME:

Babymetal are having the best year ever. Along with performing with RHCP, the group also revealed they'll be touring with Guns N' Roses in January and have a live-action/animated hybrid shortform show in the works.

Next up, check out our interview with Babymetal: