Originally Posted by Fat Organization Member XIV
Or it could be Night Slash, or something like Evil Ace. (Dark Aerial Ace, anyone?)
That's a screencap of Aerial Ace's animation in Black2/White2. Chespin's attack on Golurk matches the background precisely, and the way it slashes down and then up in a diagonal fashion matches the way Aerial Ace is animated in Gen 4 and 5.
It's 99% likely to be Aerial Ace.