Shadow Form

Shadow Form is one of Sora's forms from Kingdom Hearts.

Temporary Form
Sora turned into a Shadow Heartless after his second fight with Riku. He is only in this form briefly and can only jump and run around. However, due to Kairi's heart, he is able to recover into his human form again.

Side Effects
Since he was a Heartless, albeit temporarily, he created a Nobody that became known as Roxas. Oddly, Sora could not do a lunge attack like all other Shadows. This probably has something to do with Sora's pure heart, which is the reason he can wield the Keyblade. Heartlesses only attack beings of light, or Nobodies, but because there were only Heartlesses around, Sora couldn't attack anything. Kairi's being there can be explained by Sora's pure heart, meaning he can't attack beings of light either, meaning he can't attack at all.

While this event gave birth to Roxas, it also gave birth to the unusual Nobody, Naminé, who is not born from Sora's body and soul, but from Kairi's heart. That could be the very reason Naminé has control over Sora's memory in Kingdom Hearts Chain of Memories.

It is believed and confirmed by Tetsuya Nomura that this event triggered the side effect of creating the Anti Form when Sora was given his new clothes and Drive Form abilities a year later.