Cool. I watched the Battle of the Bands episode today on a bootleg Blu-Ray of the complete Billy & Mandy series (as the complete series was never actually released on DVD, other than the first season and maybe two and three in Australia) I got off of eBay and I couldn’t tell if the song Grim was playing was a real song or not. But I wonder…
Edit 2/12/2023: The song is “Merciless” by SCP-1000.