It's from one of three animated OPs(this one for the Heaven's Feel route) for the PS Vita port of the visual novel Fate/Stay Night Realta Nua version, animated by the awesome studio called ufotable:
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There are three routes you should go through in order: Fate, Unlimited Blade Works, and Heaven's Feel.
The first two I mentioned were already adapted by DEEN but to most fans it didn't do much justice. Heaven's Feel has yet to be adapted, but there is a prequel to Fate/Stay Night called Fate/Zero which takes place 10 years before and is also animated by ufotable (and is SOOOO good too). You can probably watch Fate/Zero first but it might get a bit confusing, though you can just watch all six eps of Fate/Zero: Onegai! Einzbern Soudanshitsu after watching it to help you out. The recommended order would be reading the visual novel first then watch Fate/Zero since most of the surprises in Fate Stay Night will be spoiled by watching Zero from what I've been told. You can find a walkthrough playlist on Xerblade's youtube channel(with voices!) and keep a tab open for this as you read along to make sure you don't miss any scenes even if they're brief. Make sure to read the prologue before starting any of the routes.
Oh and the Kara no Kyoukai film series animated by ufotable takes place in the same fictional universe to some extent if you want more information on things like Origin and the Root, which don't get much explaining in Zero. There's also Tsukihime(manga is godly but DON'T watch the so-called anime) if you want to know more about Dead Apostles, it takes place in the same universe as Stay Night and Zero too.