A story idea I can't seem to crack...

The Chosen One had fled this place screaming and he hadn't been seen since. The Beast still needed killing but she didn't care about any of that. She wasn't brave. She had never been brave, but before the Chosen One had ever been chosen he had been her brother and she had loved him and she always would. She needed to know what it was about this place that had shaken him so badly.

What awaits anyone who dares to enter this place? It is where you meet your double, but not your exact double. It is where you encounter your better self, the version you always hoped you'd become and yet never managed to achieve. You meet this person and you learn the awful truth:

Only one of you can leave here alive...