Shakugan no Shana S

Invaders from the Crimson Realm have descended upon Earth looking for something called the Power of Existence. Whoever possesses this power will control the very fabric of existence. To the rescue comes an otherworldly 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! (Source: VIZ Media)

It takes place after the original ending and focuses on Shana and Yuuji in new world Xanadu.