Shakugan no Shana: The Girl With Fire in Her Eyes (Light novel)

Invaders from the Crimson Realm have come to Earth looking for something called the Power of Existence. Whoever possesses this profound power will control the very quintessence of reality. To the rescue comes an enigmatic young girl with fire in her eyes. She may be cute and petite, but she's endowed with enough preternatural strength to save the entire universe. Plus, she's got a really big sword!.