From the latest pdf:

“Let me explain,” he says to you. “It is not a physical key. It never was.
The ‘key’ was the sense of wonder, fantasy, and creativity that sparked
my imagination when I was young. These are things we do not value in
maturity. It is only natural the key to these kinds of dreams should be
lost with age.”

This game has secretly always been Changeling: the Dreaming.