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I think it's a tear between the realms of light and darkness, appearing when Ansem SoD decided to do his grand scheme, with the Pureblood Heartless taking advantage of this to break into the Realm of Light and appearing in KH1 alongside the Emblems.I don't think there is anything of super deeper significance to this. It's basically an explanation as to why Aqua was stuck in the Realm of Darkness. Fans would ask why her bonds/heart didn't provide a way similar to how Riku and Mickey and Sora were able to get out, and the Secret Episode provided an answer -- her wayfinder/bonds DID provide an exit, but the darkness snuffed it out before Aqua could use it. With her only known exit now gone, she was stuck in there.