The Oubliette

The Oubliette was a so-called “inescapable prison” in one universe. It had obsidian walls and no interior light sources.

Jenny Everywhere and her companion Laura were once thrown into the Oubliette by a group of guards. As soon as the latter were out of earshot, Jenny lit her pocket-lighter, and she and Laura began discussing ways to escape. Contrary to Laura's initial hope, this Jenny was not able to use her shifting powers as a teleportation method, as she couldn't easily teleport within a single universe, and needed time to prepare to shift between worlds. However, she did have plan: (PROSE: The Oubliette) she telepathically communicated across time with her counterpart in this world, who lived around the time when the complex containing the Oubliette was built. She then bribed the architect to build a secret escape route, which would only open if Jenny made an elaborate gesture with her hands in front of the right wall while humming a particular tune. (PROSE: The Oubliette: Solution)