February 4, 20196 yr Its previous incarnation, Anti-Form, was explained as Sora falling into the darkness and becoming a Heartless at the end of KH1, even though it was him intentionally releasing his heart, and then being given his new Drive powers at the start of KH2. This is essentially a variation on that for due to the new way his powers manifest in KH3. This one is a bit more controllable and instead of being random, it's something that he can go into when he's in extreme danger. There is a bit more to it, and I'll put it in spoilers below, so I suggest continuing watching/learning about/playing KH3 before you read: Spoiler In the final fight of the game, against Xehanort in Scala ad Caelum, Xehanort uses the power of the χ-Blade to strip Sora of his light, forcing him into Rage Form. Essentially this form, much like his Anti-form is the manifestation of the darkness within his heart that only can seize control when he gives in or it is forced out. The anti-form was confirmed to be because he turned into a Heartless, as mentioned before, and that's why he looks so much like a humanoid Heartless in it.
Seems like it just shows up with no explanation