... Long ago, there was a war about which was stronger, light or darkness. It took a lot of lives... during the war, 13 darkness and 7 lights were made to create the ultimate weapon, the X-Blade.
[Pronounced like Chi-Blade.]
Xehanort was fascinated by this history. So he took Ventus, and separated all the darkness from him, leaving only light. After making both sides stronger, he planned to put them together again, so they'd form the X-Blade... even if it meant destroying Ventus' mind and body.