You, Sonic, and Tails set off for the castle to find the mysterious "Ifrit" Eggman spoke of. The castle was haunted, and darkness reigned. And then, at some point, you became separated. Sonic and Tails took a different path, leaving you alone.

As you walked through the castle, you suddenly heard a giggle behind you. It was Sonic's voice, but a little ominous. Turning around, you yelped in fear. It was Sonic, but with a slightly greyish tint, and his eyes were red, without pupils. His grin was a little scary, his small fangs were visible.

That definitely wasn't just Sonic. It was a ghost masquerading as him.