Anomalous Dimensional Prison Cage

As the firelight slowly dies out, Charlotte watches nervously in the dim, cold cell as the back of the evil spirit leader keeps twitching.

The next moment, the leader suddenly turns around, hooks a finger into its eye socket, and slowly lifts open its own skull.

“I will grant you new insight.”

Charlotte is overjoyed and hurriedly takes out the sorcery tome hidden in the eyeball, wanting to share his knowledge as well.

A few seconds later, the leader, its mind shattered, cries and crawls out of the cell, fleeing through the arches that are gradually collapsing into endless darkness.

...

Short version:

This is a death game where lives are the stake, and a carnival for fanatics who pursue forbidden knowledge.

Charlotte, suffering from a terminal illness, also chooses to join this feast of madness in order to cure his incurable disease.