Summon Night: Twin Age - Determination ~Final~ 

The theme for the battle against the final boss. I'd describe it to you, but this is a story-driven RPG. All I WILL say is that it's a doozy.
Music's composed by Kengo Sakurai. Summon Night as a property is owned by Bandai Namco, although this game was published by Atlus in North America and Banpresto in Japan.