Mortal Symphony - Invocation Absolutely fantastic piece. Love it.
Don't know if anyone pointed out to you exactly, but the particular song they're referencing is from the mod that comes bundled with Serious Sam TSE (the classic edition, not the HD remake) called Seriously Warped Deathmatch, and not the vanilla game. I couldn't tell you which map it's from as it's been a long time since I've played it, but that's where it's from.