Keys to the Demon Prison

The concept adds a unique twist to the usual good vs evil battles. I'm curious whether this world-building came from classic myths or original imagination.

The story seemed a bit slow at the start, but once the adventure began, I couldn't put it down. Was that the intention to build suspense early on?

The abilities felt well-balanced and fitting for their personalities. I wonder if there was a specific process or if it evolved during writing.