Pencil!Tale is an AU where all the characters are in charge of both creating, as well as maintaining different parts of the core. Although the Core was originally created by W.D. Gaster, the monsters of the Underground have contributed to the story of the Core.
Although not all monsters souls are not as powerful as the boss monsters, they create enough energy to create a Story. Not only is a Story the most powerful object you can obtain, it contains the essence of the monster who created the story. Put simply, if you have a monster's Story, you control the monster's powers, actions, etr.