The Mountain was a spaceship resembling a large metallic mountain.
It was programmed to have its own intelligence by its creators, but lacked an imagination. When the crew of the ship told each other stories, the ship did not understand them, but enjoyed listening. After an illness killed the crew, the ship landed on Earth, somewhere in England, and a small village was built around it by the people of the area. The ship programmed a minute part of itself into the minds of the children born to the village, allowing it to hear the stories and imaginations of the locals. Over time, it began to understand the stories, and wanted to create one of its own before it died.
The ship harnessed the programming in the children's minds and summoned them to it, embracing them in electric cables and programming them into its central matrix. The ship used the children's imaginations to create a story of its own, and required the last child left in the village, Brynn, to finish it. With the encouragement of the Tenth Doctor, Brynn finished the story and the ship died in peace, allowing the children to return to the village. (PROSE: Once Upon a Time)