That's called Anti Form. Random chance of Sora turning into it when you try and Drive (unless you're trying to Drive into Final Form). There's an invisible "Anti Gauge" that goes up the more you Drive, and lowers by Driving into Final, Master, and various cutscenes.
Your attacks are really bloomin' fast, though. It annihilates bosses.
Your attacks are really bloomin' fast, though. It annihilates bosses.