This is a video of musician Rob Scallon (previously: playing a part of every Metallica song in order in four minutes and playing a one-string electric guitar made from a shovel) playing a metal medley on a 22-string harp. It's not ALL harp though, there's some background instruments too because harps aren't very metal all by themselves. I'm just saying, any instrument you see religious frescos of angels playing probably aren't going to be trying to punch you in the dick in a mosh pit.
Keep going for the video, but skip to 0:50 unless you're into music videos that start with a minute of dubious acting and no music.
Thanks to DD, who agrees the most metal instrument has been and always will be an electric bass made from the bones of your enemies.