SPOILER ALERT: If you haven't seen the series and plan to, DO NOT watch this video.
The song is 10th Man Down by Nightwish. Damn this chick has awesome vocals. And the band can dish out some pretty badass metal riffs. You know that when the awesome riffs start kicking in, some crazy shite is about to go down! The lyrics fit very well with Berserk. Very well indeed. The opera intense vocals and the aggressive metal riffs of the music make it a very fitting combination with the anime.